Activation energies for grain growth of AZ31 Mg alloy at different temperatures of static annealing after equal channel angular pressing (ECAP) were investigated. ECAP was conducted for an extruded AZ31 alloy with an initial grain size of 48 μm. Stability of grain structure at elevated temperatures was investigated by annealing a sample of alloy in a range of temperatures from 473 to 748 K. It was found that grain growth occurs in the range 473-673 K in unrecrystallized condition while it occurs in the range 673-773 K in recrystallized condition.