The purpose of the research work was to provide a chemical composition for mould slag for casting alumina alloyed steel having a viscosity at 1300 oC lower than 0.25 Pa·s and a liquidus temperature below 1250 oC. Furthermore, SiO2 content should not exceed 10 mass% and B2O3 should be avoided according to the state of the art research. To achieve this goal, CaO-Al2O3 based slags with addition of SiO2, Na2O, Li2O, CaF2, MgO, K2O, TiO2 and SrO were investigated. FactSage was utilized to find a composition, which would satisfy the aforementioned requirements. Then, samples were prepared, heat treated at 1400 °C and quenched to room temperature by casting on a steel plate. The samples were investigated by simultaneous thermal analysis, X-ray diffraction and scanning electron and rheometer. Additional single experiments were performed using Furnace Crystallization Test, Inclined Plane Test, Single Hot Thermocouple Technique and quenching liquid slag to 900 °C. From the results the following conclusions could be drawn. In this slag type the substitution of some CaO by SrO has a negative effect on lowering viscosity and the liquidus temperature. The viscosity increases with rising SrO content owing to the larger ion radius of Sr2+. The liquidus temperature increases as well. Increasing MgO content to substitute CaO increases the liquidus temperature due to the formation of an additional high melting phase which might be periclase (MgO). CaF2 decreases the viscosity only in a limited content range. In this range lager structure units can be broken to single units, CaF2 may work as a network modifier. Otherwise CaF2 works as a network former generating an Al-F-Al bond by dipole – dipole interaction, which increases the viscosity. For liquidus temperature a minimum was observed at 10.95 mol% CaF2. Finally, Li2O is the most effective network modifier per unit weight. It could drastically decrease the viscosity due to its small ion radius, which causes low inner friction in liquidus state. Contrary to the FactSage results, rising Li2O will decrease the liquidus temperature. So far, the best result was achieved with composition No.21. According to the simultaneous thermal analysis the sample was totally liquid from 1236 °C on, which is satisfactory. However, its viscosity of 0.283 Pa·s at 1300 °C is too high. Increasing the Li2O content may decrease the viscosity.