Abstract: Based on the differential equation of material transportation by diffusion,the mathematical model of free CBM escaped from coal reservoir by diffusion is built and the processes in different geological conditions is simulated.The results indicate that the diffusion is one of important ways of CBM escaped from coal reservoir and its speed depends on the matural history of coal organism and the characters of reservoir and cap.The closure of hydrocarbon consistency acted an important role to slow down the diffusion escape of CBM.