The effect of cobalt on the sulfiding temperature of CoO-MoO3/Al2O3 was investigated by means of temperature programmed sulfiding (TPS). MoO3 supported on alumina transforms into MoS2-x by sulfidation via formation and subsequent hydrogenation of Mo oxi-sulfide or MoS3. The temperature of which the Mo oxi-sulfide or MoS3 phase are hydrogenated (Ps temperature) decreased with increasing amount of cobalt in the CoO-MoO3/Al2O3 catalysts. A decrease in the Ps temperature was also observed in the case of ’Co/Al+Mo/Al catalysts’ which were prepared by mechanical mixing of CoO/Al2O3 and MoO3/Al2O3. On the other hand, the Ps temperature did not change by mechanical mixing of bulk CoO and the MoO3/Al2O3 catalyst. The cobalt adsorption test suggested that the cobalt species preferably interact with the molybdenum species during impregnation. It was concluded that the well dispersed cobalt species have a capability to provide spillover hydrogen to accelerate the hydrogenation of the Mo oxi-sulfide. Furthermore, a correlation between the Ps temperature and HDS activity for light gas oil was obtained. This result suggests that the cobalt species facilitate the hydrogenation of sulfur atoms adsorbed on the HDS active sites (coordinatively unsaturated Mo sites) by supplying spillover hydrogen and consequently accelerate the HDS reaction.