The effects of active phase and composite support compositions on activity of the HDS catalyst were investigated. Direct and interactive effects of USY/gamma-Al2O3 support composition, total metal (Ni + Mo) loading, molar ratios of Ni/(Ni + Mo) and EDTA/Ni on activated catalysts sulfur content and hydrodesulfurization activity were investigated using response surface methodology (RSM). 27 catalysts with surface area, pore volume and BJH adsorption (desorption) average pore sizes in the range of 199 to 277 m(2)/g, 0.50 to 0.63 cm(3)/g and 88 (75) to 107 (90) angstrom were prepared. The concentration of sulfur remained in the hydrocarbon product, treated with different catalysts, was measured and results were in the range of 10 to 37 ppm. Based on the developed models, the most active catalyst was determined to be the one with composite support containing 10 wt% USY zeolite, loaded with 18 wt% metal oxides with Ni/(Ni + Mo) and EDTA/Ni molar ratios of 0.417, and 2, respectively. The lowest sulfur content of the hydrotreated gasoil blend was predicted to be 2.3 ppm, which was experimentally validated to be 3.1 ppm.