NiMo/Al2O3 catalyst and GaNiMo/Al2O3 catalysts with Various Ga contents were prepared in order to investigate the role of Ga as a secondary promoter for NiMo bimetallic catalyst system. The hydrodenitrogenation (HDN) reaction of pyridine as a catalytic probe was conducted over these catalysts under the same reaction conditions. These catalysts were characterized by applying infrared spectroscopy (IR), temperature programmed reduction (TPR), electron spin resonance (ESR), and NO chemisorption. Gallium introduced to the NiMo/Al2O3 as a secondary promoter enhanced the catalytic activity, and in particular, the GaNiMo/Al2O3 catalyst with the gallium content of 3 wt% obtained the highest activity. Some indicative pieces of evidences corroborating such reaction results were obtained from various characterization analyses; i.e., the addition of gallium to NiMo/Al2O3 catalyst enhances the reducibility of Mo, creates more anion vacancy sites, and increases the edge dispersion of MoS2-like structure. Altogether these promotional effects give rise to the enhanced HDN conversion of pyridine of GaNiMo/Al2O3.