Abstarct: In recent years the use of natural gas as the primary fuel for vehicles has increased dramatically in Iran. In the initial investigation it is found in cases (for example selecting optimum pressures for low and medium storages), there is potential to increase efficiency of CNG stations. In this thesis, the author is looking for ways to improve the performance of these stations with thermal and thermodynamic simulations of CNG stations. In this research station, the main sections of CNG station, including compressor, storage tanks, filling process to NGV cylinder have been studied. For determination the thermodynamic properties of natural gas, AGA8 equation of state is used. The first and second laws of thermodynamics have been employed as theoretical tools in order to investigate filling process and final in-cylinder and in-reservoir conditions of a CNG station. The results showed that ambient temperature has the big effect on refueling process, chiefly final NGV cylinder and storage bank conditions. Also it is found that, the compositions of natural gas have big effects on the CNG filling process and final in-cylinder values.The results also showd that the initial pressure of cascade storage bank has big effect on the storage bank volumes for bringing up the NGV cylinder to its target pressure. The charged mass for the Pars gas (lowest methane percentage in the composition with 87% methane) and the Khangiran gas (highest methane percentage in the composition with 98% methane) are 7.5 and 9.1 kg respectively. This shows the significantly of natural gas compositions on the charged mass (21% here). It should be pointed out that the charged mass is directly related to driving range of a NGV. Also, the result shows the initial pressure of storage bank has a big effect on the storage bank volume for bringing up the NGV cylinder to its target pressure (20MPa). The volumes of low, medium and storage bank that required for bringing up the NGV on-board cylinder to its final pressure (20MPa) for the various initial cascade storage bank pressures, RP=1.025 (PLSB=4.62275MPa, PMSB=12.6075MPa and PLSB=20.5MPa), RP=1.05 (PLSB=4.7355MPa, PMSB=12.915MPa and PLSB=21MPa) and RP=1.1 (PLSB=4.961MPa, PMSB=135.3MPa and PLSB=22MPa) were found. For RP=1.025 the result also shows the filling time is about 130 seconds higher than CNG station with reservoir system with unchanged conditions. For various Iranian natural gas compositions, the optimized LPR occurs between to . Also the optimized non-dimensional MPR occurs near . At these points, the exergy destruction during filling is minimized.