Exhibit 10.1

                              FIRST MODIFICATION TO
                           FOURTH AMENDED AND RESTATED
                                 LOAN AGREEMENT

         This First  Modification  to Fourth Amended and Restated Loan Agreement
(the "First  Modification")  is made and entered into  effective May 1, 2005, by
and among  Natural Gas Services  Group,  Inc.  ("Borrower"),  Screw  Compression
Systems, Inc. ("Guarantor"), and Western National Bank ("Lender")

                                    RECITALS

         WHEREAS,  Borrower,  Guarantor,  and Lender  entered  into that certain
Fourth  Amended  and  Restated  Loan  Agreement  dated March 14, 2005 (the "Loan
Agreement"); and

         WHEREAS,  Borrower  ,  Guarantor,  and  Lender now desire to modify the
definition of "Consolidated  Current Ratio" as set out in the Loan Agreement and
to modify certain other related provisions of the Loan Agreement.

         NOW  THEREFORE,  in view of the foregoing and in  consideration  of the
mutual covenants and agreements hereinafter contained,  Borrower, Guarantor, and
Lender hereby agree as follows:


                                    ARTICLE 1
                         Modification of Loan Agreement
                         ------------------------------

         The Loan  Agreement  is hereby  amended on the  effective  date of this
amendment in the following respects:

1.       Paragraph 1.1 Defined Terms is amended in the following respects:

         "Consolidated  Current  Ratio"  means  the  ratio of (i) the sum of the
         current assets and restricted cash of the Borrower and its Subsidiaries
         to (ii) the sum of the  current  liabilities  of the  Borrower  and its
         Subsidiaries, all determined on a consolidated basis."

2.       Paragraph 6.1(a) Consolidated Current Ratio is amended in the following
         respects:

         "(a) Consolidated Current Ratio. Permit the Consolidated Current Ratio,
         as defined  herein and calculated  pursuant to Exhibit S hereto,  to be
         less than 1.4 to 1.0 as of February 28, 2005, and as of the end of each
         month thereafter."






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<PAGE>

3.       Exhibit S -  Consolidated  Current  Ratio  Calculation,  Paragraph 1 is
         amended in the following respects:

         "1.      Sum of current assets and restricted  cash of Borrower and its
                  consolidated Subsidiaries"


                                    ARTICLE 2
                                  Miscellaneous
                                  -------------

1.       The  provisions  of this  First  Modification  to  Fourth  Amended  and
Restated Loan Agreement  shall be binding upon and shall inure to the benefit of
the parties hereto and are  incorporated by reference into the Loan Agreement as
if set out verbatim therein.

2.       The Loan Agreement, as amended herein, is hereby ratified,  adopted and
confirmed  by  Borrower  and  the  Guarantor.  Each  agreement,  representation,
warranty and covenant  made by Borrower and  Guarantor in the Loan  Agreement is
hereby ratified, adopted, and confirmed by Borrower and Guarantor on the date of
execution hereof.

3.       The effective  date of this First  Modification  to Fourth  Amended and
Restated Loan  Agreement  shall be May 1, 2005, at which time the  provisions of
this First  Modification  to Fourth  Amended and Restated Loan  Agreement  shall
become operative and are incorporated into the Loan Agreement.

THIS WRITTEN  AGREEMENT  REPRESENTS THE FINAL AGREEMENT  BETWEEN THE PARTIES AND
MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL  AGREEMENTS  BETWEEN THE
PARTIES.


         Executed on May 5, 2005, but effective as of May 1, 2005.



                                      BORROWER:
                                      ---------

                                      Natural Gas Services Group, Inc.


                                      By: /s/ Stephen C. Taylor
                                         ---------------------------------------
                                         Stephen C. Taylor, President






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                                      GUARANTOR:
                                      ----------

                                      Screw Compression Systems, Inc.


                                      By /s/ Paul D. Hensley
                                        ----------------------------------------
                                        Paul D. Hensley, President



                                      LENDER:
                                      -------

                                      Western National Bank


                                      By /s/ Scott A. Lovett
                                        ----------------------------------------
                                       Scott A. Lovett, Executive Vice President



















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