The viscosities of CaO-SiO₂-Al₂O₃-MgO-FeO slags were measured under conditions of C/S=1.15-1.6, 10-13 mass% Al₂O₃, 5-10 mass% MgO and 0-20 mass% FeO. Slag viscosity decreased with increasing FeO content at a fixed basicity (CaO/SiO₂) of slag. Slag viscosity at low FeO (<7.5 mass% FeO) exhibited a minimum value by increasing MgO content in slag. Viscosity decreased with increasing slag basicity up to 1.3 while it increased as slag basicity increased from 1.3 to 1.5. It was proposed that the driving force for the decrease of slag viscosity would be an increase in depolymerization of silicate network at C/S≤1.3, while the viscous behavior at C/S>1.3 would be increased with increasing the chemical potential of primary solid phase e.g. dicalcium silicate. A thermodynamic approach for the activity of primary solid phase in molten slags resulted in a reasonable relationship between viscosity and slag components. Therefore, it was confirmed that slag viscosity in basic slags (C/S>1.3) could be estimated by the chemical potential of dicalcium silicate.