Abstract: Micro-arc oxidation(MAO) ceramic coating was formed on the AZ91 magnesium alloy substrate in silicate alkaline electrolytic solution employing a constant-current controlled mode, and the microstructure of the MAO coating was analyzed using transmission electron microscopy. The relative properties were also tested. The results show that the products formed during the micro-arc oxidation were mainly nanocrystalline MgO and MgAl2O4 in silicate alkaline electrolytic solution, and a little amorphous substrate and some coarse grains of MgO, MgAl2O4 and MgSiO3 with the size of 100-300 nm. The formation of the fine grains was attributed to that the MAO was a fast sintering and fast cooling process. The corrosion rate of the obtained oxide layer in the neutral salt atmosphere for 36 h was less than 0.09 g·cm-2·d-1, and the microhardness was above 1 000 HV.