Abstract: In order to control large calcium aluminate inclusions in bearing steel, the quantitative relationship between refining slag composition and inclusion type is discussed by calculating the formation thermodynamics of MgO·Al₂O₃ and CaO·6Al₂O₃ in this paper. The results show that MgO·Al₂O₃ can be reduced to CaO·6Al₂O₃ by dissolved aluminum with 0.10×10^-6 Ca and less than 10×10^-6 Mg in molten steel. When the basicity of refining slag is 7.04 and the content of (MgO) is 1.38%, the content of Mg in liq-uid steel is about 56% lower than the critical value, and the main inclusions is CaO-Al₂O₃, whose dimension is larger than 10μm. As the basicity of refining slag is 3.75 and the content of (MgO) is 3.14%, the content of Mg in liquid steel is about 14% lower than the critical value, and the main inclusions is MgO·Al₂O₃ surrounded by MnS, whose dimension is less than 8μm. Under the condition that the Ca/Al ratio is between 1.8 and 2.0, MgO·Al₂O₃ inclusions can be kept stable rather than reduced to CaO·6Al₂O₃ when the basicity of refining slag is between 4.5 and 5.5 and the content of MgO is between 3% and 5%.