Series of W-incorporated CoMo/gamma-Al2O3 catalysts were characterized with TPR, DRS, ESR, and WS. Two series of catalysts with varying content of tungsten were prepared for characterization by changing the impregnation order of cobalt and tungsten to a base Mo/gamma-Al2O3 catalyst. The activity promotion by relatively low content of tungsten arose from the roles of tungsten in changing the Mo-oxide coordination from tetrahedral to octahedral, facilitating the reduction of Mo-oxide species, and increasing the dispersion of MoS2. By incorporation of tungsten at a content as much as 0.025 in W/(W + Mo) atomic ratio, the MoS2 dispersion of CoMo/gamma-Al2O3 catalyst was considered to be maximized without noticible detriment to the active Co-Mo-O phase, resulting in the maximum activity promotion. The formation of the Co-Mo-O phases was more favored in the catalysts prepared by impregnating W onto CoMo/gamma-Al2O3 than in those by impregnating W onto Mo/gamma-Al2O3 before impregnation of Co. The effect of tungsten on the dispersion of active phase was not discriminated between the two series of catalysts. The activity decrease observed in the catalysts containing higher content of tungsten originated from the increase in the W-oxide coverage on the surface of Mo-oxides or Co-Mo-O phases, resulting in not only impeding the reduction or sulfidation of the oxidic precursor but facilitating the formation of less active Co-W-O at the sacrifice of more active Co-Mo-O phase.