The principle aim of this paper is to understand the crystallization of coal ash slags and the effects on the viscosity by means of high temperature viscosity measurements, in combination with Fact Sage modeling, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Four coal ashes with the fusion temperatures between 1130 and 1470 degrees C applied in entrained flow gasifiers in China were prepared for this study. The thermodynamic modeling was carried out using FactSage 6.2 software to predict the composition of homogeneous liquid slag systems as well as heterogeneous slag systems. It can be concluded that the viscosity of coal ash samples is closely related to the phases at high temperatures. The viscosity increases significantly until the mass percentage of the solid phases reaches a certain value (15.15-33.82%). For the coal ash samples enriched in Al2O3 and SiO2 with high AFT, mullite is the first solid phase forming in the liquid slag above 1600 degrees C, followed by quartz, anorthite, and ferro-cordierite, until the molten slag finally transforms to a 100% solid state. For the coal ash samples enriched in CaO and Fe2O3 with relatively lower AFT, anorthite is the first solid phase forming and separating from the liquid slag, followed by the ferrous aluminosilicate. The crystallization temperature of solid phases, as well as the crystallization rate, is determined by the chemical composition of coal ash samples. The XRD findings were further supported with Fact Sage thermochemical modeling. The Krieger-Dougherty equation combining with the Watt and Fereday model was used to simulate the viscosity results, which provided a good fit for coal ash samples with a low proportion of crystalline phases.