Abstract: The influence of rare earth oxides finely dispersed on the martensitic transformation and the mechanical properties of cobalt was studied in this paper. It was found that the martensitic transformation of cobalt was restrained and the α-Co content tensile strength, hardness and elongation of cobalt were increased by properly adding CeO₂、La₂O₃ and Y₂O₃.The largest ineremnt of above items were: α-Co 151.8%, σ_b18.7%, HRA14.3% and σ80.4% In addition, the present study found that the α-Co content of samples was reduced by quenching in water and meanwhile the strength and elongation were reduced correspondingly. But when the rare earth oxides were added in cobalt, the detriment effects of water quenching on cobalt were mitigatel and so the decrement of above items dropped down obviously.