In this study, the thermal oxidation rate of oxygen ion (O+) implanted 4H-silicon carbide (SiC) was investigated. And the critical breakdown electric field (E-breakdown) and capacitance-voltage (CV) curve of the grown oxide (SiO2) film were also evaluated. It is found that the thermal SiO2 growth rate on 4H-SiC (0001) face was significantly improved by O+ implantation. E-breakdown test results showed that the unconsumed amorphous and damaged crystalline layer under the grown SiO2 contained defects leading to an inferior critical breakdown field. Thus, in order to obtain a high-quality SiO2 film, the oxidation process should be designed delicately so that the damaged layer by implantation could be fully consumed.