MODIFICATION AGREEMENT

Exhibit 10.5

                                                      Revolving Line Credit Note

                                                           dated January 3, 2005

                             MODIFICATION AGREEMENT

This Modification Agreement ("Modification Agreement") is effective as of May 1,

2005. The parties to the Modification  Agreement are Natural Gas Services Group,

Inc. ("Borrower") and Western National Bank ("Lender").

                                    RECITALS

On January 3, 2005,  Borrower  executed  and  delivered  to Lender that  certain

Revolving  Line of  Credit  Promissory  Note in the  original  principal  sum of

$2,000,000.00,  bearing interest at the rate stated therein, with a stated final

maturity  date of January 1, 2006 (the  "Note"),  pursuant to that certain Third

Amended and Restated  Loan  Agreement  dated  January 3, 2005,  as  subsequently

amended and restated in 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

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) six percent (6.0%), 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) six percent (6.0%),  or (b) the Highest

         Lawful  Rate,  in each case  calcu