Electrochemical reactions between blood and metal electrodes have been studied since 1928. Little is known about the actual current density induced during the reaction. In this study, an in situ continuous monitoring method was developed to detect the progress of thrombosis on an oxidized 316L stainless steel coil was deployed inside the artery. Three stages of current density were observed on a 316L wire passivated with polycrystalline oxide film. In contrast, no significant current density was detected for a wire passivated with amorphous oxide film. Results showed that this in situ electrochemical monitoring method is sensitive to the passivated film on the stainless steel coil and could provide efficient and reliable information on the control of thrombosis on cardiovascular devices.