Abstract: Mere measurements can not meet the actual requirements for determining the slag viscosities. Based on the viscosity model and regular solution model for slag proposed by D Sichen and S Seetharaman, the functional relationship between the slag viscosity and composition and the Gibbs viscous activation energy and mixture free energy of slag components was derived, i.e., the viscosity estimation model. Then, the viscosities of both CaF2-CaO and Al2O3-CaO slag system samples were estimated and measured. The results indicated that, the estimated viscosities modified by the model and the measured ones coincide well for CaF2-Ca O slag system. As for the Al2O3-CaO slag system, the estimated viscosities conform basically with the measured ones but both coincide better in the vicinity of compositional eutectic point.