Change size of film on LY12 aluminum alloy in process of micro-arc oxidation was examined by propeller micrometer and eddy current thickness meter.The ceramic coating with 30 μm in thickness was prepared in 100g/L Na2SiO3 solution for 75 min with voltage of 550V and current of 25A.The phase constituent

morphology and element distribution in the ceramic coating were analyzed by XRD（X-ray diffraction）

SEM（scanning electron microscope）and EDS

and formation mechanism of phase constituent in the coating was described.The results reveal that micro-arc oxidation can increase specimen size and decrease substrate size

and the coating thickness growing to outside is equal to the substrate lossing thickness.The coating which mainly contains O

Si and Al element is made of the substrate and amounts of cavities with about several microns in diameters.Phase constituent is composed of Al phase and Al2O3 as well as SiO2 amorphous phase.Micro-hardness of the coating makes HV 2200.