The effects of severe plastic deformation through high-pressure torsion (HPT) on the microstructure and tensile properties of a biomedical Co-Cr-Mo (CCM) alloy were investigated. The microstructure was examined as a function of torsional rotation number, N and equivalent strain, epsilon(eq) in the HPT processing. Electron backscatter diffraction analysis (EBSD) shows that a strain-induced martensitic transformation occurs by the HPT processing. Grain diameter decreases with increasing epsilon(eq), and the HPT-processed alloy (CCMHPT)for epsilon(eq)=45 exhibits an average grain diameter of 47 nm, compared to 70 mu m for the CCM alloy before HPT processing. Blurred and wavy grain boundaries with low-angle of misorientation in the CCMHPT sample for epsilon(eq) <45 become better-defined grain boundaries with high-angle of misorientation after HPT processing for epsilon(eq)=45. Kernel average misorientation (KAM) maps from EBSD indicate that KAM inside grains increases with epsilon(eq) for epsilon(eq)<45, and then decreases for epsilon(eq)=45. The volume fraction of the epsilon (hcp) phase in the CCMHPT samples slightly increases at epsilon(eq)=9, and decreases at epsilon(eq)=45. In addition, the strength of the CCM samples increases at epsilon(eq)=9, and then decrease at epsilon(eq)=45. The decrease in the strength is attributed to the decrease in the volume fraction of e phase, annihilation of dislocations, and decrease in strain in the CCMHPT sample processed at epsilon(eq)=45 by HPT. () 2015 Elsevier Ltd. All rights reserved.