To analyze the correlation between the Ni species dispersion and the surface active sites, we prepared a series of Ni/gamma-Al2O3 samples by conventional impregnation method (Cl), conventional deposition method (CD), hydrothermal impregnation method (HI), and hydrothermal deposition method (HD). It was found that the number of surface active sites of the four samples with a similar Ni loading follows the order: H-D (13.2 mu mol g(cat)(-1)) > H-I (10.3 mu mol g(cat)(-1)) > C-I (8.6 mu mol g(cat)(-1)) > C-D (7.3 mu mol g(cat)(-1)). The samples with the highest Ni oxide dispersion (H-I) and the lowest Ni oxide dispersion (C-D) are all not of benefit to increase the number of active sites. Because the high dispersion results in most Ni ions embedding into the inner part of the support and the low dispersion suppresses the exposure of active sites, the Ni oxide particles with an appropriate size are helpful to obtain more active centers. (C) 2010 Elsevier B.V. All rights reserved.