Ceramic coating was fabricated on AZ91D magnesium alloys by micro-arc oxidation in silicate system which was added Al2O3 colloid, influences of adding time and amount on chemical composition, microstructure and corrosion resistance of the MAO ceramic coatings were studied. The results showed that the ceramic coating which was formed by adding Al2O3 colloid at about 6min (the total oxidation time was 10min) performed the best corrosion resistance; and the corrosion rate was reduced evidently with the increase of Al2O3 colloid dosage, however, the performance of corrosion resistance was hardly influenced when used excessive dosage