Formation behavior of anodic oxide films on magnesium in fluoride electrolytes was investigated with attention to the effects of anodizing voltage and aluminum content. In the range of voltage between 2V and 100V, porous film was formed in alkaline fluoride solution associated with high current density at around 5V and at breakdown voltage. The critical voltage of breakdown to allow maximum current flow was approximately 60V and relatively independent on substrate purity. The films formed at breakdown voltage showed a lava-like porous structure similar to those obtained on aluminum and other valve metals. Barrier films or semi-barrier films, which were composed of hydrated outer layer and relatively dense inner layer, were formed at the other voltages. In the case of AZ91D, the critical voltage increased to 70V and peculiar phenomenon at 5V was not observed, so that only barrier films were formed at less than the breakdown voltage. These phenomena can be explained by the effects of aluminum incorporation into the film to prevent dissolution and to promote passivation of magnesium. The depth profiles of constituent elements showed that aluminum distributed in whole depth of the film.