In the gasification, burnout coal particles converts to molten ash called slag with hot gas atmosphere. The slag layers which are accumulated slag particles and consist of solid and liquid phase prevent damages from hot synthesis gas and particles, and liquid slag flows down for continuous coal ash discharge. Critical viscosity temperature (Tcv) is the interface point determined by flowability between two slag layers. Higher Tcv with coal affects thick slag layer onto the gasifier wall, while for the lower Tcv, the layers may become thin or liquid flow only except to fixed solid phase. This study evaluates the theoretical influences of Tcv, critical viscosity and flux addition on the slag behavior in a commercial coal gasifier using a numerical model. The results showed that a lower slag Tcv led to an exponential decrease in the solid slag thickness owing to an increased wall heat flux and a reduced temperature difference between the slag and the coolant. However, liquid slag thickness was relatively small.