The influence of hydrothermal modification of /gamma-Al2O3 on the properties of NiMo/gamma-Al2O3 catalyst was investigated in this paper. The experimental results showed that the use of the modified gamma-Al2O3 in the preparation of the NiMo/gamma-Al2O3 catalyst led to the increase of the dispersion of the surface Mo and Ni oxides, favored the formation of the poly-molybdates and promoted the reduction of the active Mo oxides owing to the increase of the surface acidity of the modified gamma-Al2O3. Therefore, the NiMo/gamma-Al2O3 catalyst supported on the modified gamma-Al2O3 exhibited a higher hydrodenitrogenation (HDN) activity than that supported on the untreated gamma-Al2O3 in the temperature range of 300-340 degrees C. (C) 2007 Elsevier B.V. All rights reserved.