Abstract: The morphology of micro-arc oxidation coatings on AZ91D magnesium alloy was observed by scanning electron microscopy.The corrosion resistance of the coatings with different thicknesses was examined by neutral salt spray testing and electrochemical impendence spectroscopy.The results show that the compactness of the coatings declined and their quality degraded with the increase of coating thickness.The coating corrosion resistance did not increase monotonously with the coating thickness.With given technical parameters,there was a micro-arc oxidation coating with the best corrosion resistance.The best thickness was dependent on the effective thickness of the coating instead of its total thickness.In this study the corrosion resistance for the specimen with thickness of 15 μm was better than that of 6 μm,10 μm,20 μm.