Ultrafine-grained materials often possess superior mechanical properties owing to their small grain size. The high-pressure torsion (IIPT) process is a severe plastic deformation method used to induce ultra-large strain and produce ultrafine grains. In this study, the grain refinement mechanisms in the Co-28Cr-6Mo (CCM) alloy, evolution of dislocation density as a result of HPT and its effects on mechanical properties were investigated. The dislocation density and subgrain diameter were also calculated by X-ray line profile analysis. The microstructure of the CCM alloy subjected to HPT processing (CCMHPT) was evaluated as a function of torsional rotation number, N and equivalent strain, epsilon(eq). Strain-induced gamma ->epsilon transformation in neighboring ultrafine grains is observed in CCMHPT processed at epsilon(eq) = 2.25 and epsilon(eq) = 4.5. Low-angle crystal rotation around the  fcc direction occurs in different locations in the same elongated grain neighboring ultrafine grains, which suggests the formation of low-angle grain boundaries in CCMHPT processed at epsilon(eq) = 2.25 and epsilon(eq) = 4.5. Two possible grain refinement mechanisms are proposed. The maximum dislocation densities, which are 2.8 x 10(16) m(-2) in gamma phase and 3.8 x 10(16) m(-2) in epsilon phase, and maximum subgrain diameters, which are 21.2 nm in gamma phase and 36 nm in epsilon phase, are achieved in CCMHPT processed at epsilon(eq) = 9. IIPT processing causes a substantial increase in the tensile strength and hardness owing to the grain refinement and a significant increase in the volume fraction of epsilon phase and dislocation density.