A series of NiMoNx/gamma-Al2O3 catalysts with various Ni contents were prepared by a topotactic reaction between their corresponding precursors NiO.MoO3/gamma-Al2O3 and NH3. The catalysts were characterized using BET, XRD, and H-2-TPR techniques, and the HDN activity of pyridine over these catalysts was tested. XRD patterns show that metallic Ni, Mo2N and a new phase of Ni3Mo3N exist in NiMoNx/gamma-Al2O3 catalyst. H-2-TPR studies indicate that the presence of Ni lowers the reduction temperature of the passivated surface layer of nitrided Mo/gamma-Al2O3. The HDN activity for NiMoNx/gamma-Al2O3 is much higher than that for NiMoSx/gamma-Al2O3. The nitride catalyst with about 5.0 wt% NiO and 15.0 wt% MoO3 in its precursor has the highest specific denitrogenation activity. The appearance of Ni3Mo3N and the synergy between metallic Ni and nitrided Mo are probably responsible for the high activity of NiMoNx/gamma-Al2O3 catalyst. The role of Ni in HDN reaction was also investigated. The activities decrease in the order: reduced Ni/gamma-Al(2)O(3)greater than or equal to nitrided Ni/gamma-Al2O3>partially reduced Ni/gamma-Al2O3 and sulfided Ni/gamma-Al2O3.