The dehydriding processes and kinetics of nanocrystalline hydrided Mg and AZ31 Mg alloy powders at different temperatures have been studied. By theoretical analysis of the experimental data of the pyrolytic dehydriding reaction and fitting them with the Johnson-Mehl-Avrami (JMA) equation, the kinetic equation and the corresponding kinetic parameters including the reaction order, the reaction rate constant, and the reaction activation energy have been obtained. Furthermore, the reason why there is difference in dehydriding kinetics between the hydrided Mg and AZ31 Mg alloy powders has been interpreted referring to the thermodynamic and kinetic factors.