The viscosity-temperature relation of slag determines its behavior inside an entrained flow coal gasifier. However, existing prediction models give results with large variations among them. We investigated the influence of different viscosity models in the prediction of the steady and transient behaviors of slag flow on the wall of a gasifier undergoing gas temperature changes. Five viscosity models adopted showed differences in the temperature (T250) at 25 Pa.s as large as 97 K for the selected slag composition, which was used as the interface temperature between the solid and liquid slag. When the predicted viscosity and corresponding T250 increased, the solid slag became thicker and the dynamic response of the slag became slower. In contrast, the differences in the liquid slag thickness were small. The influence of T250 predicted was dominant, compared to that of different viscosity curves of the liquid slag.