Abstract: The viscosity model, which has already been built to apply as a reference to the slag systems without SiO2, was used to estimate the viscosities of Al2O3-CaO system. The results showed that the estimated values conform well with the measured values as shown in some known literatures. The ionization factor, L, between A1³⁺ and Ca²⁺ in the Al2O3-CaO system was determined from the experimental data as shown in known literatures. According to the measurements, the slags have good fluidities around the eutectic point and the viscosities are below 0.5 Pa·s at 1460°C for Al2O3-CaO system. As to Al2O3-CaO-MgO system, the slags were prepared according to compositions of two different eutectic points, and the viscosity of the slag which contains more CaO is lower, ie. lower than 0.03 Pa·s at 1500°C. In addition, the additive MgO can reduce obviously the slag viscosity values of Al2O3-CaO system.