An unsupported Ni-Mo-W catalyst was synthesized by the hydrothermal method and treated by an aging treatment to improve its specific surface area and pore size. The effect of different aging conditions was studied by BET, XRD, SEM, and HRTEM techniques. The catalytic activity of the unsupported catalyst was evaluated with a simulated diesel feed. BET results showed that the catalyst specific surface area and pore volume increased significantly after aging treatment, and the hydrotreating results revealed that aging treatment resulted in higher catalytic activity than untreated catalyst, which is beneficial to the production of Ultra-Low Sulfur Diesel (ULSD).