A long-term reliability of thermal oxides grown on n-type 4H-SiC(0001) wafer by dry oxidation has been investigated. In order to examine the effects of SiC wafer quality on the reliability of thermal oxides, two types of 4H-SiC epitaxial wafers with different influences, which were surface roughness and metal impurity concentration, were used. A barrier height of oxide/4H-SiC interface derived from Fowler-Nordheim plot was about 2.8 eV on the both wafers. Time-zero dielectric break-down measurement results showed that almost thermal oxides ruptured at the field-to-breakdown (E-BD) of 10 MV/cm, and maximum EBD was 11 MV/cm regardless of the influences of SiC wafer. However, it was indicated that the charge-to-breakdown (Q(BD)) of thermal oxide depended on the influences by time-dependent dielectric breakdown measurements. The Q(BD) value of 63% cumulative failure for the thermal oxides grown on SiC wafer with low influences was 0.16 C/cm(2), which was about one order higher than that for high one. These results show that two types of oxide breakdown regimes would exist under high field region that E-BD is more than 10 MV/cm, resulting in the difference of Q(BD) between both wafers.