Abstract: By taking the theoretical study of temperature rise within a hydrogen cylinder during a fast filling process and the complexity of a fast filling process with real hydrogen into consideration, a high-precision simulated model of a fast filling process had been build based upon the k-ε turbulence model of real gas. Computational fluid dynamics (CFD) software Fluent 13.0 was applied to simulate the refueling process within a filament wound composite hydrogen cylinder with aluminum liner. In order to study the effect of the length to diameter (L/D) radio and the inlet diameter of the cylinder on temperature rise and temperature distribution, numerical simulations were performed with different L/D radios such as 3.6 and 2.0. And to ascertain the effect of inlet diameter, cylinders with the inlet diameters of 16mm, 40mm, and 64mm were simulated as well. The results show that the larger ratio of L/D, the higher temperature rise within the cylinder. Similarly, the temperature rise within a larger inlet diameter cylinder is higher than that within a smaller inlet diameter cylinder.