At CNG fuelling station, natural gas is usually stored in a cascade storage system to utilize the station more effi cient. The cascade storage system is generally divided into three reservoirs, commonly termed low, medium and high-pressure reservoirs. The pressures within these three reservoirs have huge effects on the performance of a CNG fuelling station and a fast fi lling process of natural gas vehicle's (NGV) cylinder. A theoretical analysis is developed to study the effects of the reservoirs pressures and temperatures on the performance of the CNG station. The analysis is based on the fi rst and the second law of thermodynamics, conservation of mass and ideal gas assumptions. The results show that as the reservoir temperature decreases, the fi ll ratio increases and the pressure within the fi lling station reservoirs has no effects on the fi ll ratio. The non-dimensional entropy generation and fi lling time profi les have opposite trends and as entropy generation decreases, the fi lling time increases. The optimized non-dimensional low and medium pressure-reservoir pressures are found to be as 0.24 and 0.58 respectively in thermodynamic point of view.