Co-Cr alloys, which have an excellent strength and wear resistance, are used as biomaterials in applications such as artificial joints. In this study, the corrosion resistance of Co-Cr alloy was investigated by electrochemical measurements. Type 316 Stainless steel, Ti, Co, and Cr were used as comparison materials. According to the polarization curves obtained in this study, the Co-Cr alloy maintained a passive state over a wide potential region, same as that observed for Cr. However, localized corrosion of type 316 stainless steel occurred to in the passive region of Co-Cr alloy. The passive current density of Co-Cr alloy was constant independent of NaCl concentration. The passive film on Co-Cr alloy, which has high localized corrosion resistance, was stable against chloride ions of high concentration. Open circuit potential of the Co-Cr alloy changing with immersion time appeared to be similar to that of Cr. As a result, the reactivity of Co-Cr alloy was considered to be dominated by Cr. Therefore, the Co-Cr alloy is expected to be safely used as biomaterials.