In this study, the correlation of surface oxidation and Mg-based intermetallic phases in the grain boundary in Al-Mg alloys containing third elements was investigated. The experimental results were examined by phase diagrams plotted as a function of oxygen partial pressure determined by thermodynamic calculation. The addition of Si and Cu as third elements into the Al-7 mass%Mg alloy formed Mg-based secondary phases during solidification. The 1 mass% Cu addition formed three different types of Mg-based intermetallic compounds. From weight gains by oxidation, all samples exhibited their weight gains depending on time. The Si-added alloy showed a considerably lower weight gain and maintained a nearly constant weight, while the weight gain of the Al-7 mass%Mg-1 mass%Cu alloy was significantly greater than those of other alloys. MgO and MgAl₂O4- spinel were the main oxides that formed the oxide scale in all examined alloys. Si addition formed the multi-element oxide including Mg and Si.