As an important and inevitable component of gasification slag, residual carbon is an internal factor that influences the flow properties of slag. In this study, the effect of residual carbon on slag viscosity was investigated. Viscosity of slag containing graphite was measured at 1350-1680 °C under an argon (Ar) atmosphere. The fusibility of ash was examined with thermomechanical analyzer (TMA). X-ray diffraction (XRD) was used to investigate the mineral composition of ash processed at high temperatures and that of the slag after viscosity measurement. FactSage was employed to calculate the free energy (ΔG) of the reactions within ash and slag at high temperatures. The slag viscosity increased with increasing the mass fraction of residual carbon, and a sharp increase of viscosity with decreasing temperature was observed for slags containing residual carbon. The presence of residual carbon led to the formation of crystalline phases, and it had a significant effect on slag viscosity when the mass fraction exceeded 5%. Quartz and mullite was the two main crystalline phases for slags with less than 5% residual carbon. However, when the mass fraction of residual carbon was more than 5%, moissanite as well as quartz and mullite were formed in slags. Ash fusibility increased with increasing the mass fraction of residual carbon for the same reason. Thus, less than 5% of residual carbon within slag is desirable in order to prevent the above problems. FactSage calculation revealed that SiO2 preferred to react with CaO and Al2O3 to form anorthite and mullite at high temperatures. Moissanite was produced by carbothermic reaction among anorthite, mullite, free SiO2 and residual carbon. Furthermore, SiO2 became difficult to react with graphite as it was chemically combined with CaO and Al2O3. Due to the low ΔG of carbothermic reaction between SiO2 and residual carbon, coals with high S/A ratios are favorable to the carbothermic reaction due to the abundant free SiO2 in the ash and slag.