The anodizing process for Mg alloys investigated in alkaline NaOH solutions with additives as a non-chromate method, and corrosion resistance in NaCl solution. A special high-current-density phenomenon was observed at 3 V-7 V in the anodizing. The Mg(OH)2 film was formed at 3 V The amount of Mg(OH)(2) increased with decreasing potential, resulting in improving corrosion resistance. The MgO film was formed at 80 V and corrosion resistance was lower than that at 3 V. Current density after anodizing at a constant potential decreased with increasing Al content in Mg-Al alloys. While the relative intensity of Mg(OH)(2) decreased with increasing applied potential, that of MgO increased. The corrosion current in NaCl solution for the Mg-15 mass% Al alloy anodized for 10 min was increased by pitting on the beta phase. However, the specimen anodized for 30 min has better corrosion resistance because the film formed on the beta phase was compact.