Abstract: The formation of CaO-TiO₂-MgO-Al₂O₃ dual phase inclusion in 321 stainless steel was investigated in the laboratory. The result indicated that the condition for the formation of CaOTiO₂-MgOAl₂O₃ in 321 steel is Ca>0.001wt%, Ti>0.1wt%, and Al>0.01wt%. The mechanism is the following: Al₂O₃ inclusion turns into CaO-Al₂O₃ after Ca-Si wire is fed into the molten steel; Mg is then obtained by reducing MgO in slag or crucible wall by Al and Ti; finally CaO-Al₂O₃ inclusion is changed into CaO-TiO₂-MgO-Al₂O₃ by the reaction with Mg, Ti, and O in the molten steel simultaneously.