Al(2)O(3) thin film was formed on pure magnesium and its anodic coating by sol-gel method. SEM observation and EDX analysis showed that Al(2)O(3) film could not totally cover the surface of base metal, but they blocked small pores when they were formed on anodic coating. Potentiodynamic polarization test indicated that Al(2)O(3) films improved corrosion resistance of both anodizing coating and base metal, and the effect on anodizing coating was better than on base metal.