Abstract: The effect of water vapour on the thermal decomposition of manganese carbonate was studied with the help of DTA and TG in different atmospheres. The kinetic equations and apparent activation energy for the decomposition reactions of manganese carbonate in dry/wet air and dry/wet 1hydrogen were deduced by the method of non-isothermal kinetics. It has been found that all these processes were diffusion controlled;the apparent activation energy were 173.7, 154.9 and 211.9, 173.7kJ.mol^-1 respective-y.According to the experimental results, it believed that the role of water vapour is principally to catalyse the decomposition reaction of manganese carbonate to manganous oxide and carbon dioxide.