Plasma electrolytic oxidation (PEO) coatings were prepared on magnesium alloy AZ91D in a phosphate based electrolyte with different additives such as sodium fluoride (NaF) and sodium aluminate (NaAlO 2 ). The effects of F − and AlO 2 − additives on the morphology, phase structure and corrosion resistance of PEO coatings were examined by scanning electron microscopy (SEM/EDX), X-ray diffraction (XRD) and potentiodynamic polarization. The results show that PEO coatings produced in F − additive solution grow faster than that in AlO 2 − additive one. However, the growth rate decreases with treatment time in F − additive solution while it increases in AlO 2 − additive one. It is also found that the corrosion resistance is greatly improved after PEO treatment, and coatings produced in F − additive solution show lower corrosion current density than that in AlO 2 − additive solution.