To clarify the grain size effect on mechanical property in fine grained pure Mg, mechanical properties of fully recrystallized Mg with fine grain sizes were investigated. The specimens having fully recrystallized microstructures with mean grain sizes of 2.8 μm and 7.8 μm were fabricated by HPT processing and subsequent annealing. The 2.8 μm specimen showed discontinuous yielding with a high yield stress and low strain hardening while the 7.8 μm specimen showed continuous yielding with a low yield stress and high strain hardening. The as-HPT processed specimen showed a larger ductility than that in the subsequently annealed specimens.