CoMo/Al2O3 catalysts were prepared by impregnating Cobalt nitrate solution into oxidic or sulfided Mo/Al2O3. The properties of CoMo/Al2O3 catalysts were characterized by XRD, TPS, oxygen chemisorption and ESR. Catalytic activity of CoMo/Al2O3 catalyst was evaluated by thiophene HDS as a probe reaction. When CoMo/Al2O3 catalyst was prepared by impregnating Cobalt nitrate solution into sulfided Mo/Al2O3, the interaction between Mo and alumina became weaker and the formation of synergic phase was facilitated. These structural changes may explain higher HDS activity of CoMo/Al2O3 catalyst prepared by impregnating Cobalt nitrate solution into sulfided Mo/Al2O3.