Abstract: Diffusion modes and micro mechanism of coal bed methane(CBM) in coal is analyzed.In coal seams there are the following gas diffusion modes:Fick diffusion,Knudsen diffusion,transitionary diffusion,surface diffusion and crystal diffusion.Moreover,the migration and diffusion processes of CBM in coal seams as CBM is drained are described.The effect rules of pressure and temperature on the mean molecule free walk of methane are discussed in microscope according to molecule dynamics.Besides,the micro-mechanism of enhancing the diffusion ability of methane by increasing pressure and temperature is explained.