Magnesium alloys of AZ31 containing (0.5, 1, 1.5) wt.% of initially added CaO particles were cast in air, and their oxidation behavior was studied at 450-650℃ in air. The initially added CaO particles either decomposed to dissolve in the α-Mg matrix or precipitated as Al<SUB>2</SUB>Ca along the grain boundaries of the matrix during casting. The ignition temperatures were 565.4℃ for AZ31, 608.6℃ for AZ31+0.5 wt.%CaO, and 689.7℃ for AZ31+1 wt.%CaO. No ignition ℃curred for AZ31+1.5 wt.%CaO up to 700℃, displaying good oxidation resistance. The CaO-rich oxide scales that formed on the surface of the AZ31+(0.5, 1, 1.5) wt.%CaO alloys improved the oxidation resistance of AZ31 alloys.