Crystallographic phases of Plasma electrolytic oxidized Al alloy, A1100, A5052, A6061, A6063, A7075, wereinvestigated. Two types of electrolyte Na2Si2O3 and Na2P2O7 were also compared. Bipolar pulse, 2000 μsec with 400 μsec +420 V impulse and 300 μsec − impulse were applied for 20 min. α-Alumina, γ-alumina, η-alumina, Al4.95Si1.05O9.52, and(Al0.9Cr0.1)2O3 were mainly observed. Si, component of electrolyte, were moved into the PEO layer by bipolar pulse. Glassyphase was also observed at the surface of Na2Si2O3 electrolyte treated PEO layer, and increased with the Mg content of Alalloy. It is concluded that at first glassy phase was formed by the micro plasma, and the high temperature of plasma turnsglassy phase to several crystalline phases. And we could expect that many other crystalline phase could be formed by PEOprocess.