A thermodynamic model was developed to predict slag–steel–inclusion reactions of 304 stainless steels. The dissolved aluminum in the steel, the sulfur distribution ratio and the composition of inclusions equilibrated with varying slags were predicted using the current model. The model can also be widely used to predict the liquid fraction of inclusions. For Al-killed 304 stainless steels, the optimized composition of CaO–Al₂O₃–SiO₂–MgO slags is proposed to modify solid Al₂O₃ inclusions to liquid CaO–Al₂O₃–MgO at 1873 K. For Si-Mn-killed 304 stainless steels, the optimized composition of CaO-SiO₂-MgO-Al₂O₃-20%CaF₂ slags is suggested to suppress the formation of Al₂O₃–MgO in SiO₂–CaO–MnO–MgO–Al₂O₃ inclusions and lower their melting temperatures.