Effects of Al content on oxidation behaviors of Al-containing 18 mass%Cr stainless steels, which are used for catalysis substrate in motorcycle mufflers, were investigated both in an exhaust gas atmosphere and in air at temperatures from 873 to 1373 K. The specimens with more than 2 mass%Al exhibit excellent oxidation resistance at 873-1373 K because the Al₂O₃ layer formed at the top surface acts as protective film against oxidation. Weight gain during oxidation is largest in the specimens having about 1 mass%Al in exhaust gas at temperatures higher than 1073 K. It is considered to be caused by the formation of the Cr-Al depleted zone between the Cr₂O₃ layer at the top surface and the discontinuous Al₂O₃ layer in the base metal. Red scale oxidation is observed only in the specimen with 1.6 mass%Al at 973 K in exhaust gas. This phenomenon can be explained by the same mechanism as the oxidation behavior observed in the 1 mass%Al-containing specimens at temperatures higher than 1073 K.