Abstract: In accordance with the coexistence theory of slag structure and the phase diagram, mass action concentration calculating models corresponding to various temperature ranges for CaO-SiO₂ slag system are formulated. Calculated results at steelmaking temperatures show that both calculating models, considering two silicates (CaSiO₃ and Ca₂SiO₄) and considering three silicates (CaSiO₃,Ca₂SiO₄ and Ca₃SiO₆), are applicable for mass action concentration calculation. Comparison of different calculating variants has proved that using thermodynamic data obtained by the regression analysis in this paper gives better agreement between calculated NCaO and measured aCaO than using the data from references does, so the later should be investigated in detail in the future. The correspondence of maximum mass action concentrations of silicates with their congruent melting points in phase diagram tells us that silicates play very important roles in the melting temperature of the givem system. The appearance of minimum Σn in the course of its variation with basicity can be explained by the fact that several structural units can combine to form one silicate molecule.