In order to clarify the relationship between the inhibiting effect of H2S and the promoting effects of Co and Ni on hydrodesulfurization (HDS) activity, HDS reactions of benzothiophene (BT) and dibenzothiophene (DBT) over sulfided CoMo/Al2O3, NiMo/Al2O3, Mo/Al2O3 were carried out in the stream of H-2 or 5% H2S/H-2 using a conventional fixed bed flow reactor. Both the inhibiting effect of H2S and the promoting effect of Co (or Ni) on an HDS reaction were observed in the reaction of both reactants on every catalyst examined here. HDS activity of Mo/Al2O3 was most severely inhibited by the addition of H2S. NiMo/Al2O3 was more susceptible to the inhibiting effect of H2S than CoMo/Al2O3. The promoter eased the inhibiting effect of H2S on HDS activity of these promoted catalysts. HDS of DBT was more sensitive to the promoting effect of Co and Ni, and to the inhibiting effect of H2S, than that of BT. The present results supply new points of view with respect to the promoting effects of Co and Ni in this catalysis.