The oxidation behavior of pure Mg and Mg-Gd-Y-Zr alloy was studied in O 2 at 300°C with and without the presence of water vapor. The kinetics curves revealed improved oxidation resistance of Mg-Gd-Y-Zr alloy in O 2 , compared with pure Mg. However, when water vapor co-existed with oxygen, the oxidation rate of Mg-Gd-Y-Zr alloy was accelerated; whereas, the oxidation rate of pure Mg was restrained. Detailed XPS analysis of pure Mg oxidized with water vapor revealed that the reduced oxidation rate could be strongly linked with the outer Mg(OH) 2 film. On the contrary, for Mg-Gd-Y-Zr alloy, an incomplete Mg(OH) 2 film was present in the outer region of oxide layer, which can provide a ready pathway for water vapor transport to the inner part of the oxide film and which has little oxidation resistance to water vapor.