Classic molecular gas dynamics and fractal pore models are used to simulate gas diffusion and reaction within char pores in combustion. Through numerical simulations it is found that the char combustion model based on Thiele modulus can be well used in regular pores, but is not well used in real char particles that have fractal pore structures. When oxygen concentrations are the same in different char pores, which is often happened in initial stage of combustion, fractal dimensions of pores will affect char combustion rates in exponential form. These numerical simulations can partly explain the previous experimental results.