The radiative defect centers in thermally-grown SiO2/4H-SiC structures with high-temperature post-oxidation annealing (POA) in various ambient gas, i.e. Ar, H2, and NOx, were examined using cathodoluminescence (CL) measurement. It was found that radiative centers with an extremely high luminescent efficiency were remained at the SiO2/SiC interfaces after Ar-POA and FGA. Thus, these defect centers are very stable against high-temperature annealing and reducing ambient. In contrast, NOx-POA significantly reduced amounts of the radiative defects that might be related to channel mobility improvement in SiC-MOSFETs.