EXHIBIT 10.26

                                 PROMISSORY NOTE



$2,100,000.00                   Midland, Texas                   January 3, 2005


     FOR VALUE RECEIVED,  in the manner,  on the dates and in the amounts herein
stipulated,   Natural  Gas  Services  Group,  Inc.  (the  "Maker"),  a  Colorado
corporation with principal offices at 2911 S. County Road 1260,  Midland,  Texas
79706,  promises to pay to the order of Paul D. Hensley,  an individual residing
at 3005 N. 15th Street, Broken Arrow,  Oklahoma 74012 ("Payee"),  at his address
in the City of Broken  Arrow,  Tulsa  County,  Oklahoma,  in lawful money of the
United States of America, the sum of Two Million One Hundred Thousand and No/100
Dollars  ($2,100,000.00),  together with interest on the  outstanding  principal
balance  hereof  (calculated  on the  basis of  actual  days  elapsed  in a year
consisting of 365 days) from date until maturity at a rate equal to four percent
(4.00%) per annum.

     Principal on this Promissory Note (this "Note") shall be due and payable in
three equal annual installments as follows:

          (1)  $700,000.00  shall  be  due  and  payable  on  January  3,  2006;
     $700,000.00 shall be due and payable on January 3, 2007; and

          (2) one final  installment of all remaining  unpaid principal shall be
     due and payable in full on January 3, 2008.

     Accrued and unpaid  interest on the unpaid  principal  balance of this Note
shall  be due and  payable  on the  same  dates  as,  but in  addition  to,  the
installments of principal.

     Subject to the prior written consent of Payee,  Maker may, but shall not be
obligated to, make principal payments on this Note:

          (1) in shares (only whole  shares) of Buyer Common Stock valued at the
     Cash Equivalent Amount Per Share for the purposes of determining the number
     of shares of Buyer Common Stock to be issued; or

          (2) by combination of cash and such shares.

     "Cash  Equivalent  Amount Per Share" means the average of the daily closing
prices of Buyer  Common  Stock on the  American  Stock  Exchange  for the twenty
consecutive  trading days commencing  thirty trading days before the due date of
any principal payment.

     All past due  principal on this Note shall bear interest at the rate of ten
percent (10%) per annum until such sums have been paid; provided,  however, that


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in no event  shall  interest  on this  Note  ever be  charged  or paid at a rate
greater than the maximum  nonusurious  rate  permitted by applicable  federal or
Texas law from time to time in effect,  whichever shall permit the higher lawful
rate (the "Highest Lawful Rate"). Interest shall be computed on the basis of the
actual number of days elapsed in a year composed of 365 or 366 days, as the case
may be. At all such times,  if any, as Chapter One ("Chapter  One") of the Texas
Credit Code shall  establish the Highest  Lawful Rate,  the Highest  Lawful Rate
shall be the "indicated  rate ceiling" (as defined in Chapter One ) from time to
time in effect.

     If, for any reason  whatever,  the  interest  paid or received on this note
shall  exceed the Highest  Lawful  Rate,  the owner or holder of this note shall
refund to the payor or, at the option of such owner or  holder,  credit  against
the  principal of this note such portion of said  interest as shall be necessary
to cause  the  interest  actually  paid and  retained  on this note to equal the
Highest Lawful Rate. All sums paid or agreed to be paid to the holder or holders
hereof for the use,  forbearance  or  detention  of the  indebtedness  evidenced
hereby shall, to the extent permitted by applicable law, be amortized, prorated,
allocated and spread throughout the full term of this note.

     Maker may prepay this Note in whole or in part at any time and from time to
time without the payment of any premium or fee. All prepayments  hereon shall be
applied to the installments of principal in inverse order of their maturities.

     This Note is one of the Promissory  Notes referred to in the Stock Purchase
Agreement,  dated of even date herewith, by and among Screw Compression Systems,
Inc., a Texas  corporation,  Jim Hazlett,  Tony  Vohjesus,  Payee and Maker (the
"Agreement"),  and is subject  to the terms and  conditions  of such  Agreement.
Reference  is made to the  Agreement  for a statement  of the rights,  remedies,
powers,  privileges,  benefits,  duties and obligations of Maker and Payee under
the Agreement.  Capitalized terms used herein which are defined in the Agreement
shall have such defined meanings unless otherwise defined herein.

     Payment of this Note is secured by that certain  Irrevocable Standby Letter
of Credit, dated January 3, 2005, in the face amount of One Million Four Hundred
Thousand and No/100 Dollars  ($1,400,000.00),  issued by Western  National Bank,
Midland, Texas, naming Payee as beneficiary.  Reference is made to the Letter of
Credit for a statement  of the nature and extent of the  security and the rights
and obligations of the Maker and Payee thereunder.

     Time is of the  essence  of this Note.  If Maker  fails to pay when due any
installment  of principal and such failure to pay continues for a period of five
days after Maker's receipt of written notice from Payee, the holder of this Note
may, at such holder's option, declare the entirety of the indebtedness evidenced
hereby  immediately due and payable and exercise any other  available  remedies,
and failure to exercise  any remedy  shall not  constitute a waiver at any other
time.

     If this  Note or any  installment  or part  hereof  is not paid  when  due,
whether  at  maturity  or by  acceleration,  or if it  is  collected  through  a
bankruptcy, probate or other court proceeding, whether before or after maturity,
the undersigned  agrees to pay all costs and expenses of collection,  including,
but not limited to, reasonable attorneys' fees, incurred by the holder hereof.


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     Maker  waives  notice  (including,  but not limited to,  notice of protest,
notice of dishonor,  notice of intent to accelerate and notice of acceleration),
demand,  presentment for payment,  protest,  diligence in collecting or bringing
suit and the filing of suit for the purpose of fixing liability.

     EXECUTED to be effective as of the date and year first written above.


                                                NATURAL GAS SERVICES GROUP, INC.


                                            By:  /s/ Wallace C. Sparkman
                                                --------------------------------
                                                Wallace C. Sparkman, President












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