The effect of high-pressure torsion (HPT) processing on the microstructure and Vickers hardness of Co-Cr-Mo (CCM) alloys were investigated in this study. The microstructure of initial CCM alloy contains equiaxed grains with a grain diameter of approximately 50 mu m and twins. The clear grain boundaries of equiaxed grains and twins disappear after HPT processing at a rotation number, N, of 10. The phase maps of initial CCM alloy and CCM alloy subjected to HPT processing at N = 5 measured by electron backscatter diffraction exhibit that the ratio of. phase decreases from 93.5% to 34.1% and the ratio of e phase increases from 6.5% to 65.9% by applying HPT processing. These results indicate that the e phase is formed by high-strain, which is induced by the HPT processing. The Vickers hardness values on the surfaces of the CCM alloys subjected to HPT processing at N = 1, 5, and 10 increase with increasing the equivalent strain, epsilon(eq). These results suggest that an increase of Vickers hardness is correlated to an increase of the ratio of epsilon phase and the dislocation density, and grain refinement, which are caused by the high-strain induced by HPT processing.