Oxidation behavior of CaO added Mg and Mg-5Al alloys was investigated. At $500^{circ}C$, oxidation rate of pure Mg was abruptly increased, while that of CaO added Mg was remarkably reduced with increasing CaO content. As a result of surface analysis by AES, there was the thin oxide layer mixed with MgO and CaO in CaO added Mg. Oxidation behavior of CaO added Mg was similar to that of Ca added Mg. Oxidation rate of 0.6CaO added Mg-5Al alloy was much lower than that of Mg-5Al alloy at $500^{circ}C$ and both alloys showed the linear weight change at $400^{circ}C$.