AZ91 Mg alloy was treated by solid solution and ageing heat treatment (T6), followed by plasma electrolytic oxidation (PEO). The microstructure and corrosion behavior of the T6, PEO and T6-PEO treated AZ91 alloy as well as the as-received one were investigated using scanning electron microscopy, X-ray diffraction and corrosion tests, respectively. The chemical compositions of the naturally formed oxide film on sample surface were determined by X-ray photoelectron spectroscopy. Especially, the effect of T6 treatment on the corrosion behavior of the bare and PEO treated specimens was evaluated. Results showed that the T6 treatment resulted in the formation of high density fine β-phase in α-Mg matrix and enhanced the corrosion resistance of AZ91 alloy. Meanwhile, the T6-PEO treated alloy had much better corrosion resistance than the PEO treated one, due to the combined effects of reduced defects of PEO coating and improved corrosion resistance of the T6 treated substrate.