Exhibit 10.1

                                                   Revolving Line of Credit Note
                                                           dated January 3, 2006


                             MODIFICATION AGREEMENT
                             ----------------------

This Modification Agreement ("Modification  Agreement") is effective as of March
24, 2006.  The parties to the  Modification  Agreement  are Natural Gas Services
Group, Inc. ("Borrower") and Western National Bank ("Lender").

                                    RECITALS

On January 3, 2006,  Borrower  executed  and  delivered  to Lender that  certain
Revolving  Line of  Credit  Promissory  Note in the  original  principal  sum of
$10,000,000.00, bearing interest at the rate stated therein, with a stated final
maturity date of December 1, 2006 (the  "Note"),  pursuant to that certain Sixth
Amended  and  Restated  Loan   Agreement   dated  January  3,  2006  (the  "Loan
Agreement"). All liens, security interests and assignments securing the Note are
collectively called the "Liens". Terms defined in the Note or the Loan Agreement
and not otherwise  defined  herein shall have the same meanings here as in those
documents.

At  Borrower's  request,  Borrower  and  Lender  have  agreed to enter into this
Modification  Agreement  to modify the  interest  provisions  of the Note and to
ratify the Liens. Said modified interest provisions shall be effective as of the
April 1, 2006 payment.

                                    AGREEMENT

1.  Modification  of Interest  Provisions  of the Note. In lieu of the following
provisions which were contained in the first paragraph of the Note:

         "...at a rate per  annum  which  shall  from day to day be equal to the
         lesser  of (a) a rate  per  annum  (the  "Established  Rate")  equal to
         one-half of one  percent  (0.5%) over the Prime Rate in effect from day
         to day, calculated on the basis of actual days elapsed, but computed as
         if each calendar year  consisted of 360 days, or (b) the Highest Lawful
         Rate."

such provisions of the Note are changed to read in their entirety as follows:

         "...at a rate per  annum  which  shall  from day to day be equal to the
         lesser of (a) a rate per annum (the  "Established  Rate")  equal to the
         Prime Rate in effect from day to day, or (b) the Highest  Lawful  Rate,
         in each  case  calculated  on the  basis of actual  days  elapsed,  but
         computed as if each calendar year consisted of 360 days."





                                       1
<PAGE>

2.  Ratification  of Liens.  Borrower  and Lender  further  agree that all Liens
securing  the Note  shall  continue  and  carry  forward  until the Note and all
indebtedness  evidenced  thereby is paid in full.  Borrower  further agrees that
such liens are hereby ratified and affirmed as valid and subsisting  against the
collateral described therein,  and that this Modification  Agreement shall in no
manner  vitiate,  affect or impair the Note or the Liens  (except  as  expressly
modified in this  Modification  Agreement)  and that such Liens shall not in any
manner be waived, released, altered or modified.

3. Miscellaneous.

         (a)      As modified  hereby,  the provisions of the Note and the Liens
                  shall  continue  in  full  force  and  effect,   and  Borrower
                  acknowledges  and affirms its liability to Lender  thereunder.
                  In the event of an  inconsistency  between  this  Modification
                  Agreement  and the  terms  of the Note or of the  Liens,  this
                  Modification Agreement shall govern.

         (b)      Borrower hereby agrees to pay all costs and expenses  incurred
                  by Lender in connection with the execution and  administration
                  of this Modification Agreement.

         (c)      Any default by Borrower in the  performance of its obligations
                  herein contained shall constitute a default under the Note and
                  the Liens and shall allow Lender to exercise any or all of its
                  remedies  set  forth in such  Note  and  Liens or at law or in
                  equity.

         (d)      Lender  does  not,  by  its  execution  of  this  Modification
                  Agreement, waive any rights it may have against any person not
                  a party hereto.

         (e)      All terms, provisions,  covenants,  agreements, and conditions
                  of the Note and the Liens are  unchanged,  except as  provided
                  herein.  Borrower agrees that this Modification  Agreement and
                  all of the covenants and agreements  contained herein shall be
                  binding upon Borrower and shall inure to the benefit of Lender
                  and  each  of  their  respective   heirs,   executors,   legal
                  representatives, successors, and permitted assigns.

                  THIS  MODIFICATION  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.







                                       2
<PAGE>

                                     Borrower:

                                     Natural Gas Services Group, Inc.

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


                                     Lender:

                                     Western National Bank

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


STATE OF TEXAS             ss.
                           ss.
COUNTY OF MIDLAND          ss.

         This  instrument  was  acknowledged  before  me on March 31,  2006,  by
Stephen C. Taylor,  President of Natural Gas  Services  Group,  Inc., a Colorado
corporation, on behalf of said corporation.


                                       -----------------------------------------
                                       Notary Public, State of Texas


STATE OF TEXAS             ss.
                           ss.
COUNTY OF MIDLAND          ss.

         This instrument was acknowledged  before me on March 31, 2006, 2006, by
Scott A. Lovett,  Executive Vice President of Western  National Bank, a national
banking association, on behalf of said association.


                                       -----------------------------------------
                                       Notary Public, State of Texas








                                       3