Co-Mo/Al₂O₃ Sol-Gel Catalyst have been investigated by X-ray diffraction(XRD) FTIR BET and TEM, and thiophene hydrodesulfurization(HDS) and their activities were compared with that of impregnated catalyst. A series of sol-gel catalysts were prepared by mixing Co-, Mo-acetylacetonate(Co-, Mo-acac)with alumina sol, or by impregnating dried gel or calcined gel with these precursors. The alumina sol was obtained from hydrolysis of Al(OR)₃ with excess water and peptization of the resultant hydrate with HNO₃ . The reference catalyst was prepared by impregnating γ-Al₂O₃ with aqueous solution of cobalt nitrate and ammonium molybdate. For the sol-gel catalyst formed from mixing all Co-Mo-acac with alumina sol, the HDS activity was lower than that of reference catalyst. However, HDS activity was significantly increased to the activity of the reference catalyst, when the half of Co-, Mo-acac were mixed with alumina sol and the rest of them were impregnated into calcined gel. For the catalyst first formed from alumina sol mixed with only Co-acac, followed by impregnating a Mo-acac solution into the calcined gel, HDS activity was lower than that of catalyst prepared by reversing the order of adding Co-acac and Mo-acac. This reduction of activity results from the absence of synergic effect due to the migration of cobalt into the alumina matrix during sol-gel process. The impregnated cobalt are, however, deposited on the surface of the preoccupied MoO₃ fine crystallite, so that HDS activity is increased due to the synergic effect. The sol-gel catalysts have a micropore of about 2nm in radius which affects the access of thiophene reactants to active sites, thereby lowering the HDS activity of sol-gel catalysts than that of conventional cobalt-molybdenum on gamma alumina catalyst.