Micro-arc oxide (MAO) films were prepared on AZ91D magnesium alloy in an electrolyte bath of aluminate and sodium hydroxide with different additives such as montmorillonite, EDTA and arabin. The effects of additives on the chemical composition and corrosion resistance of MAO films were examined by means of SEM, EDS, XRD and potentiodynamic polarization in a 3.5% NaCl solution. The results show that the morphology of MAO films is like alveolate, and the microstructure is consisted of MgAl2O4, MgO and Mg2SiO4 phases. Compared with the untreated magnesium alloy, the MAO treated specimen exhibits higher corrosion resistance, especially for the films using montmorillonite as additive