Abstract: Coal samples were affected by industrial microwave oven and coal porosity, adsorption volume, osmotic volume under different times were measured by using physical method, and the mechanism of microwave on coal was investigated. The results showed that the coal sample porosity and osmotic volume increased firstly and then decreased with the increase of microwave irradiation time; the coal sample adsorption volume decreased firstly and then increased with the increase of microwave irradiation time. The coal porosity, adsorption capacity and osmotic volume appeared extreme value which was more favorable for the gas migration, infiltration and proliferation at microwave irradiation time.