Mould slag plays a pivotal role in the continuous casting of steel. CaF2-containing slag compositions have been industrially successful, but associated health hazards have been demonstrated. The purpose of this master thesis is to develop a novel CaF2-free slag composition within the Na2O-K2O-CaO-MgO-FeO-MnO-SiO2-TiO2-Al2O3-B2O3 chemical system with the help of FactSage and optical basicity calculation. The compositions are expressed as molar ratios between the oxides. The requirements for the composition are to exhibit low crystallization tendency, have a melting point below 1250 oC, and viscosity below 3 poise at 1300 oC. FactSage was utilized to produce slag compositions that meet such criteria. Several compositions are tested, with a boron-free and a boron-containing compositions meeting most of the criteria. Slag compositions were mixed from pure raw material, liquefied at 1400 oC, and quenched to assess the glass formation ability. Compositions demonstrating high glass formation ability were selected for further testing. Viscosity at 1300 oC and dynamic break temperature were determined with the help of a concentric cylinder type rheometer with a cooling rate of 5K/min. Furnace crystallization test and simultaneous thermal analysis were conducted to measure the temperature where crystals are formed. The furnace crystallization test samples were used for scanning electron microscopy to determine the mineralogical composition with the aid of the XRD and SEM results. Time-temperature-transformation diagrams were constructed with single hot thermocouple technique to assess the crystallization tendency. Two novel slag compositions exhibit relatively satisfactory characteristics. One is a boron-free slag with the chemical composition (wt%) of 39.22% SiO2, 31.39% CaO, 11.53% Na2O, 5.84% K2O, 3.03% MgO, and 8.99% TiO2. Its Tm is 1161 oC, but its viscosity at 1300 oC is 4.48 poise. Its nose temperature is 900 oC, at which point t0.5 is 3.6 ± 0.8 sec, and t95 is 22 ± 3 sec. Improvements are needed in regards to viscosity. The other slag is a boron-containing composition with the chemical composition (wt%) of 37.23% SiO2, 26.66% CaO, 12.27% Na2O, 6.21% K2O, 2.81% MgO, 8.35% TiO2, and 6.47% B2O3. Its Tm is 1188 oC, and its viscosity at 1300 oC is 2.21 poise. Its nose temperature is 700 oC, at which point t0.5 is 6.1 ± 1.3 sec, and t95 is 44 ± 13 sec. The results from tested compositions suggest that introducing Al2O3 to the boron-free slag at above 4 wt% increases the crystallization tendency very significantly due to the formation of C2S, but the viscosity is reduced to 3.3 poise. Adding MnO and FeO at small percentages to the boron-free composition significantly reduces glass formation ability. However, if added together with Al2O3, viscosity is significantly reduced to 1 poise without reduction in glass formation ability.