The authors have investigated effects of thermal oxidation on deep levels in the whole energy range of bandgap of 4H-SiC which are generated by ion implantation, by deep level transient spectroscopy (DLTS). The dominant defects in n-type samples after ion implantation and high-temperature annealing at 1700oC, IN3 (Z1/2: Ec – 0.63 eV) and IN9 (EH6/7: Ec – 1.5 eV) in low-dose-implanted samples, can be remarkably reduced by oxidation at 1150oC. However, in p-type samples, the IP8 (HK4: Ev + 1.4 eV) survives and additional defects, several defects such as IP4 (HK0: Ev + 0.72 eV) appear after thermal oxidation in low-dose-implanted samples. The defects except for the IP8 center can be reduced by subsequent annealing at 1400oC. These phenomena are explained by a model that excess interstitials are generated at the oxidizing interface and diffuse into the bulk region.