A series of NiMo/Al2O3 catalysts were prepared using an aqueous solution containing Ni and Mo salts and citric acid (CA). Hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of biphenyl showed that both HDS and HYD activities of NiMo/Al2O3 catalysts were enhanced with addition of CA. The promotion effect of CA can be attributed to a morphology control of the MoS2 phase and an increase of the NiMoS phase. Transmission electron microscopy analysis indicates that CA has a minor effect on MoS2 dispersion but increases its mean stacking degree, with 3-layer dominating. X-ray photoelectron spectroscopy analysis confirmed the increase of the NiMoS phase. A preferential complexation between Ni and CA was supported by the fact that the catalyst prepared by reacting Ni with CA before Mo in the impregnation solution had the best catalytic activity and CA has no promotion effect within the monometallic Mo/Al2O3 catalyst.