Several series of Co-Mo-based carbon catalysts series were synthesized using three methods, successive impregnation, successive impregnation with sulfidation in between, and equilibrium adsorption methods. Co/C and Mo/C were also prepared by impregnation. The activities of sulfided catalysts for hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) at; models of the most refractory sulfur-containing compounds found in petroleum oil were studied using an autoclave reactor operating at 2.9 MPa hydrogen pressure. The activity results were found to be highly dependent on the method of preparation. The series that was prepared by impregnation but with the sulfidation step before the cobalt addition exhibited superior activities for hydrodesulfurization (HDS) of DBT and 4,6-DMDBT. The activity results were compared with that of the conventional commercial Co-Mo/Al2O3 catalyst. Co-Mo/C catalysts had about twice the activity of the commercial catalyst. It was found that the activity of HDS depends on the metal loading as well as the order of metal addition during preparation. The highest synergetic effect between Co and Mo for HDS activity was observed at a Co/Mo atomic ratio of 0.325.