Abstarct: In this project, the utilization of combined heat and power systems and turbo-expanders in city gate stations is investigated technically and economically. Usually, high pressure natural gas loses its pressure in city gate stations by using expansion valves, to achieve to an appropriate and reliable pressure for consumption. But, in this expansion method, a considerable amount of pressure exergy of natural gas stream is ignored. Turboexpander is a reliable technology to convert this exergy to a useful form of energy such as mechanical or electrical energy. Because of increasing in heat requirements of station by using this system, it has been suggested to utilize a CHP system to meet the requirements of the station and to generate electricity as a peripheral product. Three different options with some different scenarios are introduced to utilize in CGS. A new algorithm is utilized to simulate and modeling of this options and scenarios. These options are utilization of just an expander in CGS, to utilize just a CHP system in CGS and the last case is to utilize both of them in typical city gate stations. Some city gate stations are investigated as case studies. These stations are Birjand, Mashhad West and Mashhad East city gate stations. A new parameter is introduced that is called capability factor to forecast the operation of different CGSs with different sizes. Finally, as regards daily natural gas consumption in Iran that is reported to be about 387 million cubic meters, it is specified that the potential of utilizing these three options to generate electricity is about 24MW, 285MW and 562MW respectively, that specially the third on is a considerable amount of energy with a benefit of selling electricity about 295M$ in a year.