There are three main targets for improving CNG refueling station's performance including: filling time, filling mass and utilization ratio of the reservoir tanks. There are many researches concentrated on the thermodynamic modeling of CNG refueling station which mostly didn't address the optimal volume ratio of reservoir tanks. In the present work, firstly, the fast filling process has been modeled thermodynamically. The AGA8 equation of state has been employed to calculate the required thermodynamic properties of CNG. Secondly, filling time minimizing, filling mass maximizing and utilization ratio maximizing have been employed as the objective function for the optimization problem. Finally, the optimal volume ratio of reservoir tanks has been determined as the main goal of the present study. The results have been compared with the outcome of previous numerical works and also validated against the experimental data. An improvement in the fast filling modeling results, compared to the previous works, is evident in the present work. The conclusive results of the optimization problem are presented. The optimal utilization ratio are obtained for 0.45 < N-LPR < 0.52 and N-MPR/HPR = 2. An example for selecting the optimal volume ratio of reservoir tanks is performed to see the results more clearly. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.