This paper reports on studies of the phase composition and electrochemical performances of microarc oxidation (MAO) films formed on AZ91D alloy using step-down current method in phosphate electrolytes (P-film) and silicate electrolytes (Si-film). The results showed that the P-film was mainly composed of Mg, MgAl 2 O 4 , MgO and the Si-film was composed of Mg 2 SiO 4 , MgO. There clearly existed a fluoride-enriched zone of about 1∼2 μm for P-film and 0.7∼1 μm for Si-film at the MAO coating/substrate interface. The electrochemical tests showed that both P-film and Si-film could enhance the corrosion resistance of AZ91D magnesium alloy significantly; the corrosion failure processes of the two films in 5 wt.% NaCl solution were quite different.