The corrosion properties were investigated for extruded Mg–3Al–1Zn (AZ31) alloy in alkaline NaCl solution which was manufactured by adding 0.5 wt% CaO without SF6 protective gas. The results of electrochemical tests showed that the addition of 0.5 wt% CaO drastically increased the pitting potential and charge transfer resistance of the specimen, indicating that CaO addition enhances the film stability of AZ31 alloy. As results of electron probe X-ray microanalysis (EPMA), Al2Ca phase and CaO was observed on the CaO-added specimen. The Al2Ca phase was precipitated by the reduction of CaO, and was known as an effective corrosion barrier and hinders the corrosion propagation. Also, the CaO raw particle was observed, and could be remained after reduction as Al2Ca phase. Meanwhile, the Mg17Al12 phase was not recognized due to the low content of Al in the AZ31 Mg alloy. The results of X-ray diffraction (XRD) showed that Ca(OH)2 and Al2Ca phase was observed on the CaO-added AZ31 Mg alloy after immersion for 5 days, which increased the protectiveness of Mg(OH)2 film of the CaO-added specimen.