The catalytic hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrocracking (HC) of decalin, tetralin, and diphenylmethan (DMP) over Ni-Mo sulfide catalysts supported on ultrastable Y-type (USY) zeolite have been studied. The catalysts are characterized using NH3 temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), UV-vis-NIR diffuse reflectance spectroscopy (DRS), and Na and Al K-edge X-ray absorption near-edge structure (XANES). Ni-Mo sulfide catalyst supported on USY zeolite has an unusually high catalytic activity for the hydrotreating reactions of the model compounds compared with other zeolites and their supported catalysts, NH3 TPD shows the presence of a strong acid site at about 430 degrees C in USY zeolite and its supported Ni-Mo catalyst, which is dominantly characterized by Bronsted acidity. The surface concentrations of Ni and Mo in both calcined and sulfided Ni-Mo/USY zeolite catalysts are very low, indicating that the Ni and Mo phases are present in the crystal structure of USY zeolite. The diffuse reflectance spectra of calcined Ni-Mo/USY catalyst show that Mo is dominantly four-coordinate with oxygen and Ni is six-coordinate, consistent with the Ni and Mo species being present in the crystal structure of USY zeolite, Al Kedge XANES spectra of calcined Ni-Mo/USY catalyst also indicate the presence of extra-framework Al, and the content of the extraframework Al is much higher in Ni-Mo/USY than in Ni-Mo/NaY catalyst. However, there is no evidence that the extra-framework Al contributes to the acidity and the increased catalytic activity. Thus, the high catalytic HDS and HC activities of Ni-Mo/USY compared with other zeolites and their supported Ni-Mo catalysts are attributed to the synergistic effect between the strong Bronsted acid sites and the Ni and Mo sulfide phases in the sodalite cage and/or supercage of USY zeolite.