<PAGE>
 
                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549


                                  SCHEDULE 13G



                   Under the Securities Exchange Act of 1934

                              (Amendment No. 1)*


                            MATRIX SERVICE COMPANY
                                (Name of Issuer)

                                 COMMON STOCK
                         (Title of Class of Securities)

                                   576853105
                                 (CUSIP Number)

                               December 31, 1999
            (Date of Event Which Requires Filing of this Statement)


Check the appropriate box to designate the rule pursuant to which this Schedule 
is filed:

     [_] Rule 13d-1(b)
     [_] Rule 13d-1(c)
     [_] Rule 13d-1(d)


*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).

                               Page 1 of 5 pages

<PAGE>
 
CUSIP No. 576853105                     13G                   Page 2 of 5 Pages
          

 
------------------------------------------------------------------------------
      NAME OF REPORTING PERSONS.
 1.   I.R.S. IDENTIFICATION NOs. OF ABOVE PERSONS 
           
      HEARTLAND ADVISORS, INC.
      #39-1078128 
------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP 
 2.                                                             (a) [_]
                                                                (b) [_]
                                                 
------------------------------------------------------------------------------
      SEC USE ONLY
 3.
 

------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4.                                 
      WISCONSIN, U.S.A.

------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5.                                                       
     NUMBER OF            0
 
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6.       
                          None 
     OWNED BY
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7.       
    REPORTING             600,000
 
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8.       
                          None
                          
------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9.     
      600,000
------------------------------------------------------------------------------
      CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES
10.                         
                                                                    [_]
 
------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
11.           
      6.2%

------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON (SEE INSTRUCTIONS)  
12.           
      IA

------------------------------------------------------------------------------

<PAGE>
 
CUSIP NUMBER 576853105                                      Page 3 Of 5 Pages

Item 1.

     (a) Name of Issuer:  Matrix Service Company
         --------------

     (b) Address of Issuer's Principal Executive Offices:
         ----------------------------------------------- 
               10701 E. Ute Street
               Tulsa, OK 74116-1517

Item 2.

     (a) Name of Person Filing:  Heartland Advisors, Inc.
         --------------------- 

     (b) Address of Principal Business Office:
         ------------------------------------ 
               Heartland Advisors, Inc.
               789 North Water Street
               Milwaukee, WI 53202               

     (c) Citizenship:    Heartland Advisors is a Wisconsin corporation.
         -----------

     (d) Title of Class of Securities:  Common Stock
         ----------------------------

     (e) CUSIP Number:   576853105
         ------------

Item 3. If this statement is filed pursuant to (S)(S)240.13d-1(b) or 
        ------------------------------------------------------------
        240.13d-2(b) or (c), check whether the person filing is a:
        ---------------------------------------------------------

     (a) ___  Broker or Dealer registered under Section 15 of the Act 
              (15 U.S.C. 78o).

     (b) ___  Bank as defined in Section 3(a)(6) of the Act (15 U.S.C. 78c).

     (c) ___  Insurance company as defined in Section 3(a)(19) of the Act 
              (15 U.S.C. 78c).

     (d) ___  Investment company registered under section 8 of the Investment
              Company Act of 1940 (15 U.S.C. 80a-8).

     (e)  X   An investment adviser in accordance with 
         ---  (S)240.13d-1(b)(1)(ii)(E);

     (f) ___  An employee benefit plan or endowment fund in accordance with 
              (S)240.13d-1(b)(1)(ii)(F);

<PAGE>
 
     (g) ___  A parent holding company or control person in accordance with
              (S)240.13d-1(b)(ii)(G);

     (h) ___  A savings association as defined in Section 3(b) of the Federal 
              Deposit Insurance Act (12 U.S.C. 1813);

     (i) ___  A church plan that is excluded from the definition of an
              investment company under section 3(c)(14) of the Investment
              Company Act of 1940 (15 U.S.C. 80a-3);

     (j) ___  Group, in accordance with (S)240.13d-1(b)(1)(ii)(J).

If this statement is filed pursuant to (S)240.13d-1(c), check this box [_].

Item 4. Ownership.
        ---------

     (a)  Amount beneficially owned:
          -------------------------
          600,000 shares may be deemed beneficially owned within the meaning of
          Rule 13d-3 of the Securities Exchange Act of 1934 by Heartland
          Advisors, Inc.

     (b)  Percent of Class:  
          ----------------
          6.2%
                         
     (c)  For information on voting and dispositive power with respect to the 
          above listed shares, see Items 5-8 of the Cover Page.

          
Item 5.  Ownership of Five Percent or Less of a Class.
         --------------------------------------------

     If this statement is being filed to report the fact that as of the date
hereof the reporting person has ceased to be the beneficial owner of more than
five percent of the class of securities, check the following: [_]


Item 6.  Ownership of more than Five Percent on Behalf of Another Person.
         ---------------------------------------------------------------

     The shares of common stock to which this Schedule relates are held in 
investment advisory accounts of Heartland Advisors, Inc. As a result, various
persons have the right to receive or the power to direct the receipt of
dividends from, or the proceeds from the sale of, the securities. The interests
of one such account, Heartland Value Fund, a series of Heartland Group, Inc., a
registered investment company, relates to more than 5% of the class.

Item 7.  Identification and Classification of the Subsidiary Which Acquired the
         ----------------------------------------------------------------------
         Security Being Reported on By the Parent Holding Company.
         --------------------------------------------------------

     Not Applicable.     

Item 8.  Identification and Classification of Members of the Group.
         ---------------------------------------------------------

     Not Applicable.     

Item 9.  Notice of Dissolution of a Group.
         --------------------------------   

<PAGE>

     Not Applicable.     

Item 10. Certification
         -------------

     By signing below, I certify that, to the best of my knowledge and belief,
the securities referred to above were acquired and are held in the ordinary
course of business and were not acquired and are not held for the purpose of or
with the effect of changing or influencing the control of the issuer of the
securities and were not acquired and are not held in connection with or as a
participant in any transaction having that purpose or effect.


                                   SIGNATURE

          After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete and
correct.


DATE:        January 20, 2000

             HEARTLAND ADVISORS, INC.

             By: PATRICK J. RETZER
                  Patrick J. Retzer 
                  Senior Vice President