A kinetic model to predict chemical composition changes in molten steel, slag, and inclusions in ladle refining was developed and used to elucidate the mechanism underlying the change in the chemical composition of the inclusions. The coupled reaction model was applied to estimate the reaction between molten steel/slag and molten steel/inclusion originating from the slag. The thermodynamic calculation software, FactSage6.3, was employed to obtain the activity of each component in the slag phase. Empirical equations were applied to the reaction between the slag and the refractory. The resulting model can calculate changes in (1) the composition of each element in the molten steel, slag, and the inclusion originating from the slag, (2) the amount of inclusion originating from the slag and the deoxidation products, and (3) the ratio of the inclusion originating from the slag and the deoxidation products to the total inclusion. The calculated results were found to agree with the operational results of a 165 t ladle refining process reported in the literature. The deoxidation products altered from alumina to a MgO·Al₂O₃ spinel-type inclusion due to an increase in the Mg content of steel. In the average composition changes of each element in the total inclusions, calculated results for the MgO and Al₂O₃ contents were also found to agree with the operational results.