The behavior of anodic film growth on magnesium and magnesium alloys in sodium hydroxide solution was investigated with a focus on the effects of substrate composition, formation voltage, and aluminum addition. In the range of formation voltage between 2V and 110V, barrier films or semi-barrier films were formed except at around 5V and breakdown voltage. At the latter voltages, a porous type film was formed. The critical voltage of high current flow accompanied by breakdown was considerably dependent on substrate purity; namely, 60V for 99.6% Mg, 90V for AZ 31B, and 100V for AZ 91 D and 99.95% Mg. A peculiar phenomenon of high current density at around 5V may be caused by trans-passive state, similar to that associated with alkaline fluoride solution. The peak current at 5V decreased with increasing aluminum content of the substrates. These phenomena can be explained by the effects of aluminum incorporation into the film to prevent dissolution and to promote passivation of magnesium. When 1mol·dm^-3 AlO₂^-ion was added in the electrolytes, the critical voltage of 99.95% Mg increased to 110V. The depth profiles of constituent elements showed that aluminum was distributed throughout the whole depth of the film. The atomic ratio of aluminum to magnesium increased with increasing voltage to attain an atomic ratio of approximately 2/3 at 100V and crystalline MgAl₂O₄ was found in the film.