Plasma electrolytic oxide coatings were produced on both pure magnesium and binary Mg-Al alloys containing 3, 6, 7 wt% Al, and the effect of Al on the characteristics of coatings was investigated under the processing conditions of various coating times and current. The coatings on Mg-Al alloys showed porous microstructure and some volcano top-like pores on the surface, and they consist of MgO and Al2O3. The surface roughness became larger with increasing coating time, while it was unchanged with increasing Al content. The thickness increased with increasing coating time and showed slight dependence on Al content. The hardness was almost unchanged with increasing coating time and Al content in the coatings grown with the ratio of current, C1:C2=100:75.