A composite ceramic coating containing Al2O3-ZrO2-Y2O3 was successfully prepared on AZ91D magnesium alloy by plasma electrolytic oxidation (PEO) technique in an alkaline aluminate electrolyte. The morphology, elemental and phase composition, corrosion behavior and thermal stability of the uncoated and coated samples were studied by environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectrometer (EDS), X-ray diffractometer (XRD), electrochemical corrosion test, high temperature oxidation test and thermal shock test. The results showed that the composite ceramic coating was composed of Al2O3, c-ZrO2, t-ZrO2, Y2O3 and some magnesium compounds, such as MgO, MgF2 and MgAl2O4. After PEO treatment, the corrosion potential of AZ91D alloy was increased and the corrosion Current density was significantly reduced. Besides, the coated magnesium alloys also showed excellent high temperature oxidation resistance and thermal shock resistance at 500 degrees C environment. (c) 2009 Elsevier B.V. All rights reserved.