The formation mechanism of anodic oxide films on Mg alloys when anodized in NaOH solution. was investigated by focusing on the effects of anodizing potential. Al content. and anodizing time. Pure Mg and Mg-Al alloys were anodized for 10 min at various potentials in NaOH solutions. $Mg(OH)_2$ was generated by an active dissolution reaction at the surface. and the product was affected by temperature. The intensity ratio of $Mg(OH)_2$ in the XRD analysis decreased with increasing applied potential. while that of MgO increased. The anti-corrosion properties of anodized specimens at each constant potential were better than those of non-anodized specimens. The specimen anodized at an applied potential of 3 V had the best anti-corrosion property. And the intensity ratio of $Mg_{17}Al_{12}$/Mg increased with aluminum content in Mg-Al alloys. During anodizing. the active dissolution reaction occurred preferentially in ${eta};phase(Mg_{17}Al_{12})$ until about 4 mins. and then the current density increased radually until 7 mins. The dissolution reaction progressed in a phase(Mg) which not formed the intermetallic compound. which had a lower Al content. In the anodic polarization test of $0.017;mol{cdot}dm^-3$ NaCl and $0.1;mol{cdot}dm^-3;Na_2SO_4$ at 298 K. the current density of Mg-15 mass% Al alloy anodized for 10 mins increased. since the anodic film that forms on the a phase is a non-compacted film. The anodic film on the phase for 30 mins was a compact film as compared with that for 10 mins.