Contamination and a degree of deactivation of Ni-Mo/gamma -Al2O3 catalyst used for solvent hydrotreatment in a coal liquefaction process were examined based on the data obtained from a 150 ton/day pilot plant. When solvent hydrotreatment was carried out at 300 degreesC, the Fe, Ca, and Si contents of the catalyst used for 3200 h were higher than those for 500 h. The Fe contaminant tended to be greatly concentrated near the edge of the catalyst grain. Pore volumes of the catalysts used for 3200 and 500 h were 0.34 and 0.39 ml/g, respectively. The catalyst used at 325 degreesC for 2800 h contained more Fe than that at 300 degreesC for 3200 h. No difference in carbon content was obvious among the three used catalysts. In the down-flow fixed bed hydrotreater, Fe content was larger while the pore volume was less upstairs. In solvent hydrotreatment at 300 degreesC, the degree of deactivation was 20% after 3200 h. When solvent hydrotreatment was carried out at 325 degreesC, the degree of deactivation reached 25% after 2800 h. The amount of contaminants are compared in relation to the degree of deactivation, and mechanisms on catalyst deactivation in the solvent hydrotreatment are discussed.