The AZ91 magnesium (Mg) alloy is a high strength yet lightweight material, which is suitable properties for structural engineering application. However, the high surface reactivity of the alloy to a corrosive environment limits its extensive use. A barrier layer is necessary to reduce contact between the alloy and the environment. This paper reports the characteristics of a film formed on the AZ91 alloy by plasma electrolytic oxidation (PEO) in Na3PO4 solution at variation time of 30, 60, and 90 s. Surface characterization by SEM revealed that the formation of the PEO film was initiated at the β phase (30 s) and then expanded to the metal matrix (60 and 90 s). The film formed for 30 s was localized at β phase. At a prolonged time of 60 and 90 s, the film became more uniform with a thickness of approximately 2.5 and 5.0?μm, respectively. The film composed mainly amorphous Mg(OH)2 phase, as analyzed by XRD. The wear resistance, which was measured by abrasion test, showed that the film formed at the intermediate time of 60 s exhibited the highest abrasion rate of 3.22x10-4 mm3/Nm which was three times than that of the film formed at 30 and 90 s with abrasion rate of 1.35x10-4 and 1.47x10-4?mm3/Nm, respectively.