An improvement in the mechanical properties of a biomedical Ni-free Co-Cr-Mo alloy under as-cast condition has been examined by means of tensile tests and microstructure observations. The solubility of N in Co-Cr-Mo alloys increases with increasing Cr content from 29 to 34 mass%. This results in a significant improvement in mechanical properties such as yield stress, tensile strength, and fracture elongation. The improvement in the mechanical properties results from the γ phase stabilization and inhibition of the σ phase formation due to N addition. Increasing the Cr content also has contributed to the improved mechanical properties.