Abstarct: In the natural gas transmission network, the high pressure gas is moved over long distances. At the entrance of towns, natural gas pressure has to be reduced to lower level. This pressure reduction is also carried out in cities and power plants entrance stations. Currently, the pressure reduction is carried out by throttling valve while considerable amount of pressure energy in is wasted. One of the equipment which could be used to recover this waste energy is the reciprocating expansion engine. The purpose of this research is to simulate one-sided reciprocating expansion engine thermodynamically. In this study, the temperature and pressure within cylinder have been calculated numerically for each crank angle by two ways, ideal gas model and AGA8 standard . In addition, the effects of the engine geometrical characteristics, such as intake and exhaust port area and ports timing on the work output are investigated.According to results, simulated engine has 91% thermal efficiency and with the suction of 20gr gas per cycle, generate 50kW power. This engine is optimum in D_i/D=0.28and .