The contact angle between molten Fe-Al alloy with 0.03, 0.3, and 3 mass% Al composition, and Y2O3 matrix oxide substrate with 0.002, 0.32, and 1 SiO2 activity was measured using sessile drop method in Ar atmosphere at 1873 K, and the interfacial tension was evaluated. The contact angle and interfacial tension between the molten Fe-0.3 Al alloy and the Y2Si2O7 + SiO2 (aSiO2 = 1) substrate decreased over time during 60 s after the molten alloy was dropped onto the substrate. The decrease of the contact angle was 20°, and that of the interfacial tension was 628 mN・m−1 Conversely, the other contact angles and the other interfacial energies were almost stable during the same period. The decrease of the contact angles ranged between 0° and 7°, and that of the interfacial tensions ranged 4 and 195 mN・m−1. By observing the wetting behavior for 60 min, it was recognized that the interfacial reaction between the Fe-Al alloy and the oxide substrate was the redox reaction between Al composition in the alloy and SiO2 composition in the substrate, composed of SiO2 decomposition reaction and Al2O3 formation reaction between oxygen absorbed at the interface and Al composition in the alloy. In addition, it was indicated from the interfacial tension dependence on SiO2 activity that the medium SiO2 volume slag for the molten low-Al steel and the low SiO2 volume slag for the molten high-Al steel were effective in preventing the small droplets of molten slag into the molten steel.