The effects of NiMo catalyst calcination temperature (250, 350, 450, 550, and 650 degrees C) on catalytic hydrodesulfurization of dibenzothiophene (DBT) were studied. The physiochemical properties of the unsupported NiMo catalysts were characterized by various techniques, including N-2 adsorption-desorption, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). XRD and FT-IR results showed that increasing the calcination temperature brought about a phase change of the NiMo catalyst precursors from ammonium nickel molybdate to nickel molybdate. The agglomeration of precursor particles and MoS2 nanoclusters changed with various calcination temperatures as revealed by SEM and HRTEM, respectively. DBT catalytic results showed that the NiMo catalyst calcined at 350 degrees C exhibited higher activity than other NiMo catalysts, and the selectivity of bicyclohexane was much preferred at a reaction temperature of 320 degrees C over NiMo catalysts endowed with the phase of ammonium nickel molybdate. The high hydrogenation ability might be due to the efficient formation of the "Ni-Mo-S" phase and will be attractive for industrial applications.