Crystallization in the molten slag of entrained-flow coal gasifiers can increase its viscosity and affect the flow of the slag layer along the wall. The chemical composition of the slag plays an important role in crystallization behavior and crystal characteristics. The aim of the work reported here is to study the influence of Fe2O3 on crystallization tendency. Six synthetic slags with different Fe2O3 concentrations, ranging from 5 to 30 wt %, were investigated using differential scanning calorimetry (DSC) and the single hot thermocouple technique (SHTT). In addition, three of the slags were reduced in a 60:40 CO/CO2 atmosphere, and their crystallization characteristics were compared to the original slags. The time-temperature-transformation (TTT) diagrams for each slag were constructed. The Kissinger method was used to calculate the activation energy based on DSC results under cooling conditions, and the Johnson-Mehl-Avrami (JMA) equation was applied to calculate crystallization mechanisms under isothermal conditions. As the iron oxide concentration increased, a higher crystallization temperature and lower activation energy were observed, which indicates a stronger crystallization tendency. However, when the slags were in reducing conditions, the crystallization tendency declined slightly with a higher activation energy and lower crystallization temperature. The thermodynamic modeling program FactSage was applied to predict the equilibrium composition of the system. The modeling results indicate that reduced slags have a lower liquidus temperature and less species of solids, which provided further evidence of the comparative weakness of crystallization in a reduced atmosphere. The upward shift of the crystallization temperature caused by iron oxides will significantly influence the temperature of critical viscosity (T-cv) of crystalline slag.