A series of aluminum borate with various Al/B ratios were prepared by precipitation method at constant pH value. This material has a very narrow pore size distribution centered around the pore diameter of about 6 nm. It also has a large surface area and, therefore, can be used as a support for CoMo hydrodesulfurization catalysts. Hydrodesulfurization of heavy residual oils over CoMo/Aluminum borate was carried out in a trickle bed reactor at 663 K and 7600 kPa. Temperature-programmed-reduction method was used to monitor the CoMo-support interaction. The results show that the CoMo/Aluminum borate catalysts are much more active than the commercial catalysts such as GC-106 and 107 in the initial period. A correlation exists between the reducibility of the oxidic precursor catalysts and the HDS activity of the sulfided samples. The higher the reducibility, the higher is the HDS activity.