The slag viscosity is an important factor determining the operation temperature of entrained flow type of gasifiers. The temperature of critical viscosity, Tcv, for 5 crystalline slags was predicted by empirical models and Fact-Sage equilibrium calculations, and the validity of each method was tested. Two empirical models were employed: one using Th from the ash fusion test, and the other using the concentrations of 5 major components. The first model using Th over-predicted Tcv by 20~100 ℃, while the model based on the slag composition under-predicted Tcv by 80~120 ℃. In the equlibrium calculations, Tcv was obtained from the liquidus temperature. When the 4-major component concentrations were used in the calculation, the predicted temperatures were higher than the observed. The liquidus temperature was very sensitive to the concentrations of minor components, and the addition of MgO and Na2O lowered the liquidus temperature. The results with 4 major and 3 minor components most closely described experimentally observed Tcv. In the case that a chromia refractory was used, it was shown that Cr2O3 concentration in the slag also needs to be included for more accurate prediction of Tcv.