Ceramic coatings were prepared on AZ91 D Mg alloy by micro-arc oxidation(MAO) in aluminate electrolytes,with Al_2O_3 nano-additive suspending at different concentrations.Effects of nano-additive concentration on the structure,phase composition,hardness and anti-corrosion property of the MAO coatings were analyzed by scanning electron microscopy,X-ray diffraction,micro-hardness test and electrochemical method,respectively.The results revealed that Al_2O_3 nano-particles were mostly incorporated into ceramic coating chemically,transferred into MgAl_2O_4,rather than being trapped mechanically during MAO process.With the increase of Al_2O_3 concentration,the voltage-time response,content of MgAl_2O_4,hardness and anti-corrosion property increased.However,when the concentration varied from 10 g/L to 15 g/L,these behaviors and properties changed only a little.This result indicated that,after the concentration of Al_2O_3 nano-additive reaching 10 g/L,the incorporation of Al_2O_3 nano-particles turned into a saturation state,due to the complex process during MAO treatment.Therefore,10 g/L might be a proper concentration for MAO coating to incorporate Al_2O_3 nano-particles. Ceramic coatings were prepared on AZ91D Mg alloy by micro-arc oxidation (MAO) in aluminate electrolytes, with Al_2O_3 nano-additive suspending at different concentrations. Effects of nano-additive concentration on the structure, phase composition, hardness and anti-corrosion property of the MAO coatings were analyzed by scanning electron microscopy, X-ray diffraction, micro-hardness test and electrochemical method, respectively.The results revealed that Al_2O_3 nano-particles were mostly incorporated into ceramic coating chemically, transferred into MgAl_2O_4, rather than being trapped mechanically during MAO process. With the increase of Al 2 O 3 concentration, the voltage-time response, content of MgAl 2 O 4, hardness and anti-corrosion property increased.However, when the concentration varied from 10 g / L to 15 g / L, these behaviors and properties changed only a little.This result indicates that after the concentration of Al 2 O 3 nano-additive reaching 10 g / L, the incorporation of Al 2 O 3 nano-particle s turned into a saturation state, due to the complex process during MAO treatment. Beforefore, 10 g / L might be a proper concentration for MAO coating to incorporate Al 2 O 3 nano-particles.