Abstract: The changes of apparent viscosity of coal-oil slurries with temperature during direct coal liquefaction were studied. Several factors influencing the viscosity of coal slurries were discussed, which include solvent properties, coal concentrations, moisture content, macerals of coal, shear rate, temperature, swelling behavior, etc. The apparent viscosity of coal slurries was measured using a high-temperature viscometer. The results show that the solvent property is the main influencing factor at low temperature, and the apparent viscosity decreases with increasing temperature. Solvent properties and swelling behavior work together for the viscosity of slurries at medium temperature, leaving the apparent viscosity invariable. Swelling behavior becomes the main influencing factor at high temperature, and viscosity of slurries increases with increasing temperature.