Abstract: The oxidation behavior of 436 ferritic stainless steel at 900-1150 ℃under different relative humidity was studied by using synchronous thermal analyzer.The morphology, composition and structure were characterized by EPMA and X-ray diffractometer. The results show that when the steel is oxidized in moist air above 1000 ℃, the separation oxidation occurs obviously, which leads to a dramatic increase in oxidation weight gain.The oxidation behavior of 436 ferritic stainless steel in moist atmosphere at 1050 ℃can be divided into two stages according to the change of oxidation rate: the protective oxidation stage with slow oxidation rate and the separation oxidation stage with high oxidation rate.The scale of 436 stainless steel has different structure and morphology at different stages.The complex process of the scale formation in wet atmosphere was studied from many aspects.