Change of apparent viscosity of -100 mesh Akabira coal/tetralin slurry was systematically measured under conditions of hydrogen pressure of 10.1 MPa and steady reaction temperature of 723K and for a range of heating and cooling rates, stirrer revolution and slurry concentration. This was successfully done by a torque detector fitted to the stirrer of a 50 cm3 rnicroautoclave. Viscosity of 0.2 to 12 Pa s was detectable by this torque meter. Experimental results showed that 1) a sharp peak appeared in the viscosity-time curve during heating, 2) the temperature at which the peak appeared corresponded closely to that where the preasphaltene yield reached its maximum value, and 3) a similar peak, but much smaller than the above, also appeared during cooling. Further experiments were carried out for slurries of inert carbon particles, preasphaltene and tetralin or binary mixtures of tetralin and naphthalene or quinoline to simplify the reaction system. The results together with the above implied that the apparent viscosity increases with the amount of preasphaltene residing on the surface of coal particles while it decreases with deposition of the viscous coal particles onto the reactor and stirrer surface, with dissolution of preasphaltene into the solvent and with conversion of preasphaltene to lower molecularweight species.