Pure Mg was subjected to high pressure torsion (HPT) at room temperature, and the structural and hardness evolutions were studied. In addition, the grain size evolution during the hardness steady state was systematically investigated. The results indicated that the hardness initially increases with increasing the equivalent strain, reaches a maximum value of ~53 Hv, and then decreases to a steady-state level at large strains. However, although the hardness reaches the steady state, the average grain size does not reach a steady state. The evolution of the grain size is different from the evolution of the hardness during the HPT processing. The continuously decreased grain size during harness steady state may be caused by the annihilation of the dislocations during dynamic recovery or dynamic recrystallization, resulted from the temperature rising during the continuous HPT processing and the low melting temperature of Mg.