Plasma Electrolyte Oxidation (PEO) behavior of AZ91 Mg alloy was investigated in the electrolytes with/without potassium fluoride. Growth rate of coating thickness in the electrolyte containing potassium fluoride (Bath B) was much higher than that in the electrolyte without potassium fluoride (Bath A). The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium fluoride and sodium silicate consisted of MgO, MgF₂ and Mg₂SiO₄. Corrosion current density of oxide layer coated from the electrolyte with potassium fluoride was much lower than that of oxide layer coated from the electrolyte without potassium fluoride. From the result of EIS analysis, it was known that inner barrier layer in the oxide layer coated from the electrolyte with potassium fluoride had a good influence of the corrosion resistance of Mg alloy. The corrosion resistance curves of Bath B were similar to the thickness curves, indicating that the thickness of the oxide layer played an important role in corrosion resistance of AZ91 Mg alloy. The oxide layer in the Bath B containing potassium fluoride was found to be a compact barrier-type passive film in presence of fluoride ions. The existence of the dense MgO and MgF₂ in the barrier layer had a favorable effect on the corrosion resistance of the AZ91 Mg alloy formed from Bath B by PEO process.