Unsupported Ni-Mo sulfide catalysts were prepared by thermal decomposition of tetramethylammonium thiomolybdostate and nickel nitrate, which were characterized by Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric and differential thermal analysis, X-ray diffraction, N(2) physorption, elemental mapping and transmission electron microscopy. Hydrodesulfurization of dibenzothiophene was also performed in a fixed-bed reactor over the unsupported Ni-Mo sulfide catalysts. The results showed that the structures and properties of unsupported Ni-Mo sulfides depended on nickel content and decomposition conditions. Ni-Mo sulfides with type IV isotherms related to mesoporous structure were obtained. The layer and stacking degree of sulfide particles showed a decreasing trend, which leaded to a decrease of specific surface area, with increasing nickel content. All elements were homogeneously distributed, suggesting the "Ni-Mo-S" phase was formed. The addition of nickel promoted the dibenzothiophene conversion and the direct desulfurization pathway in the hydrodesulfurization of dibenzothiophene. The sample Ni(0.1)-MoS(2) gave superior hydrodesulfurization activity for dibenzothiophene to other samples. The unsupported Ni-Mo sulfide catalysts exhibited desired activity in hydrodesulfurization and therefore might find wide applications in hydrotreating of fuel oil. (C) 2011 Elsevier B.V. All rights reserved.