Medical cast and forged CoCrMo alloys were coated by the plasma nitrided process to enhance the wear resistance. The microstructures, phases and microhardness of nitrided layers were studied by AFM, SEM, XRD and a micro-hardness meter. The experimental results showed that the plasma nitrided process has shown promise in producing thicker, harder, and more wear resistant layers than the conventional CoCrMo alloys. At low nitriding temperatures, the structures appeared to be consistent with the formation of S-phase, CrN phases in a parent structure of α-(CoCr) for the cast and forged alloy. At high nitriding temperatures, both cast and forged samples appeared to form CrN and Cr₂N as well as α-(CoCr) phases. Whether cast or forged CoCrMo alloy, the thickness of the nitrided layer increased with the nitriding temperatures. Moreover, the thickness of nitrided layers for forged alloys was higher than cast alloys under the same nitriding conditions. In the case of the cast and forged CoCrMo alloys, all plasma nitrided samples showed much higher hardness than the untreated samples.