The relationship between oxidation behavior and defect density for wet and dry oxidation of 3C-SiC films epitaxially grown on Si substrates was studied. Dry oxidation is shown to be insensitive to the presence of defects and consequently results in smooth oxides, although it produces inferior electrical characteristics compared to those of wet oxidation. In wet atmospheres, preferential oxidation at the antiphase boundaries (APBs) was observed and attributed to the weakness of strained C-C and Si-Si bonds, which are formed at the APBs. This effect leads to an increase in the mean oxidation rate with increasing defect density. Simultaneously, the surface roughness increases for low defect densities but starts to decrease again when a critical defect density is exceeded. As a limiting case, the wet oxidation rate of polycrystalline 3C-SiC films was found to be four times higher than for low defect density, single crystalline 3C-SiC films. The observed oxidation rate and surface roughness is quantitatively explained by a geometrical model, which takes the interaction of preferentially oxidized APBs into account. Employing these results, deposition of microcrystalline SiC on single crystalline 3C-SiC and subsequent wet oxidation of the microcrystalline layer was performed. As a result, smooth and uniform oxide layers were formed. Transmission electron microscopy and atomic force microscopy were used in this study.