The oxidation characteristics of high Al composition Al_xGa_1－xAs layer contained in the Al_xGa_1－xAs/GaAs materials are researched, by reaction with H₂O vapor at temperature 400～500℃. Many factors are related with the procedure of oxidation, such as the Al composition, the thickness of Al_xGa_1－xAs layer,the reaction temperature and the carrier gas flow. The results show that the oxidation rate of Al_xGa_1－xAs layer is related to the Al concentration, the thickness of Al_xGa_1－xAs layer, the reaction temperature and the carrier gas flow.