The transport properties of gases are closely related to the collision process between molecules, and in particular, the cross sections significantly affect diffusion coefficient, viscosity and thermal conductivity. This work studies the diffusion coefficients of He, Ar, N2 and CH4 as well as their temperature dependence using molecular dynamics simulations. Based on the classical scattering model of simple gases combing with the simulation results, the temperature dependence of selfdiffusion cross section and its effect on diffusion coefficient are analyzed, which is helpful for the teaching and research of transport properties of gases in the course of college physics.