Exhibit 10.6

                                                         $7,521,109.00 Term Note
                                                          dated November 3, 2003


                          SECOND MODIFICATION AGREEMENT

This  Second  Modification   Agreement  ("Second  Modification   Agreement")  is
effective as of May 1, 2005.  The parties to the Second  Modification  Agreement
are Natural Gas Services  Group,  Inc.  ("Borrower")  and Western  National Bank
("Lender").

                                    RECITALS

On November 3, 2003, Borrower executed and delivered to Lender that certain Term
Promissory Note in the original principal sum of $7,521,109.00, bearing interest
at the rate stated  therein,  with a stated final maturity date of September 15,
2007, as modified in Modification  Agreement dated January 3, 2005 (the "Note").
The Note was made  pursuant to that certain  Second  Amended and  Restated  Loan
Agreement dated November 3, 2003, as  subsequently  amended and restated in that
certain  Third Amended and Restated Loan  Agreement  dated January 3, 2005,  and
that certain  Fourth  Amended and Restated Loan  Agreement  dated March 14, 2005
(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 Second
Modification  Agreement  to modify the  interest  provisions  of the Note and to
ratify the Liens. Said modified interest provisions shall be effective as of May
1, 2005.

                                    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
         the greater of (i) one percent  (1%) over the Prime Rate in effect from
         day to day, or (ii) five and one-quarter  percent  (5.25%),  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."

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  greater  of (i)  one-half  percent  (0.5%)  over the Prime Rate in
         effect from day to day, or (ii) five and one- quarter percent  (5.25%),
         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 Second Modification  Agreement shall
in no  manner  vitiate,  affect  or  impair  the Note or the  Liens  (except  as
expressly  modified in this Second  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  Second
                  Modification  Agreement  and the  terms  of the Note or of the
                  Liens, this Second 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 Second 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 Second  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  Second   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  SECOND  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.


                                       2
<PAGE>

                  THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.


                                     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 May 10, 2005, by Stephen
C.  Taylor,   President  of  Natural  Gas  Services  Group,   Inc.,  a  Colorado
corporation, on behalf of said corporation.


                                        /s/ Deborah K. Tullos
                                       -----------------------------------------
                                       Notary Public, State of Texas

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

         This instrument was acknowledged before me on May 11, 2005, by Scott A.
Lovett,  Executive Vice President of Western  National Bank, a national  banking
association, on behalf of said association.



                                        /s/ Deborah K. Tullos
                                       -----------------------------------------
                                       Notary Public, State of Texas







                                                                               3