Abstract: In this work, an optimized solution treatment condition for the as-cast Mg-6.0Y (wt.%) alloy was established, and holding at 500°C for 1 h can effectively reduce the intergranular Y-containing precipitates. Subsequently, the oxidation behavior of this binary Mg-RE alloy was systematically investigated using quasi-in-situ experiments combined with white-light interferometry. Detailed observations of three selected grains reveal a pronounced dependence of oxide layer growth on grain orientation, namely, the oxide film thickness increases as the angle between the grain surface normal and the sample normal decreases. This can be attributed to differences in atomic density and bond energy among distinct grain planes. Additionally, a comprehensive investigation of the oxide film was conducted, focusing on its multilayered structure, arrangement, and formation mechanism across these grains. The results indicated that the oxide film consists of four alternating MgO and Y2O3 layers, with the innermost continuous Y2O3 layer being the thickest and primarily responsible for the oxidation resistance. These findings established a clear correlation between grain orientation and oxidation behavior, providing valuable insights into surface stability and the design of protective oxide films in Mg-RE alloys.