Abstarct: In this research the fast filling process of automobile’s CNG cylinders is simulated and the related governing equations are extracted. The governing equations are solved by analytical method and by using the ideal gas state equation and AGA8 gas state equation , this process is simulated numerically. For the first time all constitution components of CNG is considered for simulation of the fast filling process and because of the high precision of selected equation of state, the result is useful and acceptable for fast filling process simulation. In this research the AGA8 equation of state for first time is used for simulation of CNG extracted from IRAN natural gas wells.