There are many types of non-metallic MgAl₂O₄ inclusions observed in Al-deoxidized steel coupling with Mg treatment, including single-particle MgAl₂O₄, agglomerated MgAl₂O₄, and MgAl₂O₄-MnS. Thermodynamic calculation shows that MgAl₂O₄ precipitates in the liquid phase. The phase transformation follows liquid + Al₂O₃ + MgAl₂O₄ → liquid + MgAl₂O₄ → liquid + MgO + MgAl₂O₄ → liquid + MgO with the Mg content increasing when the Al content is a constant in molten steel, and it is in agreement with the experimental results for the formation of MgAl₂O₄ in molten steel. The calculation results of various attractive forces between two particles show that the cavity bridge force plays a dominant role in the agglomeration process and results in the agglomerated MgAl₂O₄. The lattice mismatch calculation result shows that MgAl₂O₄ can provide effective sites for MnS nucleating in steel.