Purpose: This paper deals with the mold slag viscosity calculations. Design/methodology/approach: The study involved the analysis of the impact of the chemical composition and temperature on the viscosity. Calculations were performed using models Riboud - Zhao, Urbain, Kondratiev and the computer program FactSage using module viscosity. The results of this works charakterize the effect of temperature, addition of CaF2 and Na2O on the viscosity for two types of slag. Findings: Slag 1 contained CaO/SiO2 = 0.78 and the addition of Na2O, slag 2 contained CaO/SiO2 = 1.32 wihout Na2O addition. Computer simulations revealed differences in the obtained values of viscosity. The use of the classical models: Riboud - Zhao and Urbain showed that at temperature 1200°C slag 2 contains larger content of CaO has the lowest viscosity at temperature 1500°C. The addition CaF2 causes a similar effect on viscosity of both slags. The use of the Fact Sage software showed that the addition of Na2O has a large influence on the viscosity. Slag 2 CaO/SiO2 = 1.32 although more than slag 1 is obtained at temperature 1500°C higher viscosity values. Research limitations/implications: Mathematical modeling of liquid mold slag viscosity based on classical and structural models and with the use of commercial software requires verification methods using scanning microscopy and nuclear magnetic resonance. Practical implications: The computer calculations of the mold slag viscosity shown that the selection of the chemical composition of the slag is important for the species of the cast steel and the speed of casting. Results of calculations show that the change in viscosity of the slag in the process result from the content composition: CaO/SiO2, CaF2 and Na2O content. Originality/value: The results of the calculations indicate the reaction of CaF2 with Na2O, probably results of this reaction is NaF, which prevents the formation of cuspidine which has a high melting point, its presence increases the melting point of the slag.