Various transition-metal catalysts supported on gamma-alumina were prepared and applied for the hydrodenitrogenation (HDN) of pyridine. The NiMo/Al2O3 catalyst showed higher activity than the Mo/Al2O3 catalyst. On the basis of the results of reaction experiments and measurements of IR and TPR, the enhancement in the HDN activity with nickel addition could be attributed more to the improvement in the reducibility of molybdenum than to the effect of Bronsted acid site. Tungsten impregnated on the NiMo/Al2O3 catalyst as a secondary promoter enhanced the catalytic activity. From the results of various characterization analyses, the positive evidences supporting the HDN activity promotion were obtained. These are the formation of more Bronsted acid sites (FT-IR), the enhanced reducibility of Mo (TPR), the increase of octahedrally coordinated Mo6+ species (DRS), and the creation of more anion vacancy sites (ESR and NO adsorption). Apparently, all of these features are combined to give the promotional effect of tungsten in the NiMo/Al2O3 catalyst for the HDN activity.