Mg2Ni has drawn attention due to its high reaction rates, good reversibility, and possibility to be applied as a negative electrode in the Ni-MH (metal hydride) batteries. Mg2Ni samples were prepared by sintering under Ar in a stainless-steel crucible at 823 K. Johnson-Mehl equation for nucleation and growth mechanism was applied to the behaviors of hydriding and dehydriding reactions of Mg2Ni. At 571 K under 30 bar H-2, the activation of the Mg2Ni sample was completed at the number of cycles of three (n = 3). At n = 3, hydrided fractions of the sample were 0.51 (1.92 wt% H) at 0.50 min and 0.90 (3.38 wt% H) at 2.00 min. Reverse temperature dependences of the initial hydriding rate and the quantity of hydrogen absorbed for 60 min appeared from 571 K to 620 K. Hydriding and dehydriding reactions progressed by the nucleation and growth mechanism and could be expressed by the Johnson-Mehl equation.