Entrained flow gasification employs a high temperature, high pressure slagging gasifier, in which slag viscosity plays a key role in determining operating conditions. The effect of cooling rate on viscosity properties were investigated by high temperature rotational viscometer. Viscosities of two slag, which exhibited glassy and crystalline slag behavior, were determined under continuous measurement conditions. The results showed that viscosities of two slag both decreased with increasing cooling rate. When the temperature was above the liquidus temperature (TLiquidus), the difference between slag viscosities was small. However, it showed a large difference below TLiquidus, and a more obviously effect was observed on crystalline slag. Above TLiquidus, slag viscosity only depended on the bulk composition of slag which was little affected by cooling rate, resulting in the small difference between viscosities measured at different cooling rates. Below TLiquidus, slag viscosity was closely related to the amount of solid phase that was greatly affected by cooling rate. For glassy slag, mullite was not crystalized in slag during cooling, and slag viscosity depended on the compositions of slag. However, for crystalline slag, anorthite was crystalized in slag, and its amount significantly decreased with increasing cooling rate, leading to the large difference between viscosities measured at different cooling rates. Meanwhile, temperature of critical viscosity (TCV) decreased with increasing cooling rate, and a good linear relationship existed between TCV and cooling rate. Furthermore, TCV measured under equilibrium conditions was able to be predicted by the linear regression formula of the “continuous measurement” experiments data.