Oxidic unsupported Ni-Mo-W catalysts were chosen to elucidate the effect of sulfidation conditions on the catalytic performance. The catalysts were sulfided in situ by using dimethyl sulfide as sulfiding agent. The relationships between the time needed for sulfidation and the sulfiding conditions were studied by GC-analysis method. Straight-run gas oil with high sulfur and nitrogen content was used to evaluate the hydrotreating performance. The oxidic catalyst precursors and sulfided catalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). With increasing sulfiding liquid hourly space velocity (LHSV) of the sulfiding agent, the reaction time necessary for the sulfidation decreased, while the activity of the catalyst increased significantly. The higher catalytic activity might be due to the MoS2/WS2 slabs with shorter length and higher stacking number, which might contribute to the catalyst with more active sites. Sulfiding at 330 °C took the longest sulfidation time, while the catalytic activity was also the highest after sulfiding at this temperature. Furthermore, within a certain range, the sulfidation pressure had no evident effect on the catalytic behavior or activity. The purpose of this work was to provide a basis for actual production and a reference for further research.