The absorption and desorption kinetics of the metal hydrogen storage materials are related with the grain sizes to some extent. By means of the absorption and desorption kinetics equations, the effects of the grain sizes of Mg-Ni-MnO2 hydrogen storage materials on their absorption and desorption kinetics are calculated. The results show that the reaction rates of absorption and desorption are greatly different among the Mg-Ni-MnO2 hydrogen storage materials with different particle sizes; the smaller the particle size is, the better the absorption and desorption kinetic. The experimental results of the absorption and desorption kinetics of the Mg-Ni-MnO2 materials prove the kinetic equations to be rational.