The phosphorus distribution ratios between MnO (8-16 mass%)-containing CaO-SiO₂-Fe_tO slags and liquid iron were measured at 1873 K in a MgO crucible. The Addition of MnO was found to reduce the phosphorus distribution ratio at a given total Fe content and (mass%CaO)/(mass%SiO₂) ratio. The equilibrium quotient of dephosphorization reaction, (mass%P)/{[mass%P]·(mass%T.Fe)^5/2} the activity coefficient of P₂O₅, and that of Fe_tO in logarithmic form were formulated as a linear function of slag compositions. Based on these empirical formulas, the dephosphorization in hot metal and steel was thermodynamically assessed and the characteristics of double slag treatments were discussed from the mass balance relation between slag and metal with respect to phosphorus.