Heavy oil hydrodesulfurization (HDS) was carried out using dispersed catalyst which deactivated less strongly than the conventional supported catalyst. Various kinds of transition-metal compounds containing either molybdenum, tungsten, nickel, or cobalt were tested as dispersed catalyst precursors individually or in combination. Experiments were performed in a continuous trickle bed reactor under high hydrogen pressure for the feedstock of atmospheric residual oil. To recover dispersed catalysts from the reactant mixture, activated carbon was packed into the reactor on which the dispered catalytic species happened to be deposited. Of the catalytic precursors examined, the compound containing molybdenum showed the highest conversions for sulfur and asphaltenes. A large HDS activity synergism was observed for the combined catalyst system consisted of two precursors of different metal kind. The Go-Mo catalyst showed the highest activity and selectivity for HDS reaction. Selectivity for hydrocracking reaction was the highest with the Ni-Mo catalyst. The Ni-W system showed the lowest activity in most of reactions. Go-Mo was considered as the most promising dispersed catalyst system for the HDS of heavy oil. It was proved that active carbon packed in the reactor was very effective in recovering the dispersed catalysts from reactant oils and that the dispersed catalysts deposited on activated carbon were also catalytically active, resulting in higher yield particularly for HDS.