The change in apparent viscosity of coal slurries with temperature from 323 to 663 K was examined using six kinds of coals ranging from 66 to 84% C and hydrogenated anthracene oil (HAO). The measurement was performed using a closed type of concentric double cylinder viscometer equipped with a sampling line for examining the change in properties of the solid and liquid phases in the slurry. In relation to the viscosity change, the change in swelling ratio of the coals in the HAO with temperature up to 623 K was also measured with an apparatus that detected the height of the coal layer in an autoclave utilizing X-rays. The slurries of middlerank coals, Taiheiyo, Wandoan, and Illinois No. 6, presented two viscosity peaks around 450 and 570 K in the curves of apparent viscosity versus temperature, although there was no clear viscosity peak in the slurries of the lower rank coals, Estevan and Wabamun. The maximum of the second viscosity peak increased with coal rank up to 84% C, although the first viscosity peak was hardly observed in the slurry of the higher rank coal, Lingan. The theological behavior of the coal slurries up to 570 K corresponded well to the swelling behavior of the coals : slight swelling, swelling in two steps, and drastic swelling in one step for the lower, middle, and higher rank coals, respectively, On reaching 570 K, the coals began to dissolve with the start of the liquefaction reaction and then disintegrated above 620 K, leading to a decrease in the slurry viscosity. Thus, the theological behavior of the coal-solvent slurry during heating was found to vary with both the solvent-absorption characteristics and the initial liquefaction characteristics of the coal.