Ceramic coatings were prepared on 20vol% Al2O3-SiO2(sf)/AZ91D magnesium matrix composites by microarc oxidation (MAO) technique in a phosphate electrolyte. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were employed to analyze the surface morphology, cross-section microstructure and phase composition of the coated composites. The electrochemical corrosion property of coatings was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results show that the ceramic coating mainly consists of MgO and MgAl2O4. The corrosion current density of ceramic coating decreases nearly by three orders of magnitude compared with magnesium matrix composite substrate while its electrochemical impedance increases greatly. The magnesium matrix composites treated by microarc oxidation display good corrosion resistance