In situ formation of Al2O3-SiO2-SnO2 composite ceramic coating on Al-20%Sn alloy was successfully fabricated in aqueous Na2SiO3 electrolyte by microarc oxidation technology. The compositions, structure, mechanical and tribological properties of the composite coating were detailed studied by scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, hardness tester and ball-on-disc friction tester. It is found that the species originating from the Al-20%Sn alloy substrate and the electrolyte solution both participate in reaction and contribute to the composition of the coating, which results in the generated coating firmly adherent to the substrate. The composite ceramic coating can greatly improve the microhardness and tribological property of Al-20%Sn alloy. (C) 2009 Elsevier B. V. All rights reserved.