Abstract: According to the coexistence theory of slag structure as well as the measured viscosity of MnO-SiO₂,MgO-SiO, and CaO-Al₂O₃-SiOZ slag systems at different temperatures and slag compositions, calculating models of viscosity for these slag systems have been formulated.Calculated results agree well with measured results, showing that these models can correctly illustrate the relationships of viscosity with temperatures and the mas action concentrations of the structural units of these slag systems.