The influence of a high alumina refractory, glazed by the typical CaO–MgO–Al2O3–SiO2 ladle slag, on the generation of non-metallic inclusions in Al-deoxidized and Ca-treated molten steel was investigated at 1600°C. The variations of microstructure and chemistry of the glazed refractory were investigated with reaction time. Significant chemical reaction of the glazed refractory was observed due to the infiltration of liquid glaze into porous spinel matrix region in the refractory. Various non-metallic inclusions of the CaO–MgO–Al2O3–CaS system were generated in molten steel. The major inclusions were liquid Ca–Al–Mg–O–S inclusion and its mixtures with solid phase such as CaS, MgO and MgAl2O4. Thermodynamic analysis using the FactSage software was also carried out to understand the chemical reactions of the glazed refractory with molten steel more clearly.