The kinetics of the heterogenous reaction between various sizes of solid γ-MnO₂ particle （100-200 mesh） and hydrazine solution has been studied over a range of temperature from 20 ℃ to 40 ℃. The rate of reaction was determined by measurement of the volume of N₂ gas evolved from the reaction. The molar ratio of MnO₂ to hydrazine was four to one. The smaller the particle and the higher the temperature, the greater the rate constant, so that the conversion increased accordingly. The overall reaction was found to be a second-order raction. The mechanism of the reaction seems to be a bimolecular reaction between the reactive Mn-O site and N₂H₄ molecule.