In this study, the effects of constituent phases on the oxidation kinetics of an Al-7 mass%Mg alloy containing a trace of Ca were investigated. A Mg+Al₂Ca master alloy was used to add Mg and Ca simultaneously. Scheil-Gulliver cooling by thermodynamic calculation showed that the addition of Ca led to the formation of Ca-based intermetallic compounds, such as Al₄Ca and Laves C36, after solidification. Based on weight increase indicated by the oxidation test and surface analysis, it was found that the presence of Ca-based phases significantly improved the oxidation resistance and slowed down the oxidation rates. Based on the review of phase diagrams with oxygen partial pressure by thermodynamic calculation, it was thought that in the initial oxidation, MgO and MgAl₂O₄ were formed on the surfaces of the Al-Mg alloys, leading to further oxidation. The Ca-based intermetallic compounds formed Ca-Mg-Al based oxides, which possibly contributed to the formation of a relatively dense oxide layer.