The microstructures of high-temperature oxide scales on the Si-terminated surface and C-terminated surface of 6H-SiC were investigated by transmission electron microscopy (TEM). We found that mechanical polishing caused surface strains, about 100 nm in depth, on both sides of specimens. Mechanically polished specimens were oxidized at 1473 K for 20 h in air. Oxide scales of about 250 nm in thickness were formed on the Si-terminated surface and of about 400 nm on the C-terminated surface. Since the strain regions caused by mechanical polishing were oxidized, strains were no longer observed. As a result, this oxidation condition effectively removed the strains. The oxide scales were identified as amorphous silica on the Si-terminated face, while crystalline oxides and amorphous silica were observed on the C-terminated face.