Abstract：Most of gas (methane) in coal seam is in adsorbed state. At present，the attention of researchers is attracted to the relationship of the mutual action between the solid coal and the adsorbed methane because it is closely connected with the prevention and control of the burst of coal and gas in mining and the exploitation of methane. Terzaghi¢s effective stress formula can not be adapted in the calculation of the effective stress in adsorbed methane-coal system. Therefore，based on principles of surface physicochemistry and elastic mechanics，the calculation formulas on the swelling deformation of adsorption，the swelling stress of adsorption，and the effective stress in the adsorbed methane-coal system are derived. The computation solution of theory is essentially accordant with the experimental solution. The test and analysis show that the mechanical effect of the free gas pressure in pore is less important to coal seam because the porosity of coal is smaller than the porosity of soil by an order of magnitude，and that the changing law between the swelling strain of adsorption and the swelling stress of adsorption agrees with the Hooke¢s law because micropore-containing solid matrix of coal grains is separated by crevices discontinuously；and the swelling deformations of contact points of coal grains under constraint condition are similar to that in uniaxial compression
