Cobalt-chromium-molybdenum (Co-Cr-Mo)-based alloys have been used as many kinds of biomedical applications due to their excellent biocompatibility and mechanical properties. The deformation behaviors of Co-27%Cr-5.5%Mo-0.16%N alloy (in mass%) consisting of γ phase (fcc) and Co-27%Cr-5%Mo alloy consisting of ε phase (hcp) were summarized in this study. Stacking fault formation followed by evolution to ε phase with increasing strain was dominant deformation mode in γ Co-27%Cr-5.5%Mo-0.16%N alloy. Excellent tensile ductility at room temperature more than 20% was seen in γ Co-27%Cr-5.5%Mo-0.16%N alloy. However, fracture occurred under quasi-brittle manner. Then, crack propagated preferentially at interface between γ phase and strain induced ε phase, which is due to the highly-accumulated strain at the interface. While, in ε Co-27%Cr-5%Mo alloy, simultaneous activation of basal 〈a〉 slip and prismatic 〈a〉 slip is dominant deformation mode, and fracture occurred dominantly by intergranular fracture.