A mathematical model of the reaction between mold flux and molten steel has been newly developed to estimate the change in composition of the mold flux during the continuous casting of steels and to design an optimum flux composition. The model takes into account mass transfer in the molten steel and molten flux and equilibrium reaction at the interface. On the reaction, the activities of the practical mold flux components like CaO, SiO₂, Al₂O₃, CaF₂, Na₂O are estimated by a statistical thermodynamic model of slag, called cell model. Comparison of calculated composition change with data obtained in the practical casting experiment of high titanium steel showed that the mathematical model is useful for the estimation of the reaction between the mold flux and the molten steel and for the design of the mold flux composition to minimize change in composition due to that reaction.