A series of aluminum borates (AB) with various Al/B mole ratios was prepared by the precipitation method. These samples have been characterized with respect to surface areas, pore volumes, pore size distributions, thermal stabilities, and mechanical strength. The results indicated that the exhibited properties are dependent on the Al/B ratio of the material. The monodisperse pore size distributions of these samples imply that it is a true microcomposite structure rather than a mixture of the individual materials. Hydrodesulfurization (HDS) and hydrodemetalation (HDM) of heavy Kuwait atmospheric residuum over CoMo/AB catalysts were carried out in a bench-scale trickle bed reactor at 663 K and 7582 kPa. The weight hourly space velocity of residue oils was 1.5, and the hydrogen flow rate was kept constant at 300 mL/min(STP). The results showed that these catalysts are much more active than the conventional COMO/A12O3 catalyst in HDS and HDM reactions. The results of desulfurization activity are mainly interpreted on the basis of difference in dispersion and the interaction of Mo species with the support. The demetalation activity was strongly influenced by the intraparticle diffusion of metal porphyrins. A general correlation could be established between the HDS activity and the reducibility of oxidic precursor. Larger surface area and high reducibility of active metal may produce more active sites and result in high HDS activity. In addition, larger pore size can reduce the pore diffusion resistance and may produce more opportunity for HDM reaction.