The viscosity temperature characteristic of coal ash is crucial for liquid slag entrained flow gasifier. The viscosity temperature characteristics of Yueyang coal ash were studied by the RV DV Ⅲ type high temperature viscosimeter. The influences of sample mass, temperature condition and cooling rate on coal ash viscosity temperature characteristics were researched. The results show that the mass of coal ash should be at least 80 g, and the mass of slag sample after premelting should be from 40 g to 50 g. The highest temperature in test should be 200 ℃ more than the slag's flow temperature (TF). When the temperature of sample is 100 ℃ colder than the ash melting point, the heating rate could be fast. When the temperature of sample is near the ash melting point, the heating rate should be slow, and the temperature should keep constant for a certain time when it has reached the ash melting point. Based on the results of this experiment the cooling rate at 1 ℃/min is found to be reasonable using continuous cooling way.