Crystallographic phases of Plasma electrolytic oxidized Al alloy, A1050, were investigated. The electrolyte of PEO was Na 2 Si 2 O 3 and KOH. Unipolar pulse, 2000 µsec with 400 µsec + 420 V impulse, were applied for 2 min, 5 min, 15 min, and 30 min. γ-Alumina, as well as α-alumina, was main crystal phase. γ-Alumina was appeared in the beginning, then the amount of α-alumina was increased with time, but the amount of γ-alumina remained constant without any increasing. So, it is concluded that plasma gas produce γ-alumina at the first, and then γ-alumina transform α-alumina finally. During the transformation, high temperature of micro plasma gives transformation energy.