A series of alumina-aluminum phosphates (AAP) was prepared by a coprecipitation method. These materials were used as a support of CoMo hydrodemetalation (HDM) catalysts. The samples were characterized by X-ray diffraction, BET surface area, and mercury penetration pore volume measurement. Hydrodemetalation of Kuwait atmospheric tower bottom (ATB) residuum over the sulfided catalysts was carried out in a cocurrent down-flow trickle bed reactor at 663 K and 7.6 MPa. The hydrogen flow rate was kept constant at 300 mL(STP)/min and the ATP flow rate at 0.5 mL/min. A second-order rate equation fits reaction data. The apparent activation energies for dinickelation, devanadization, and demetalation are around 24 kcal/g-mol. All catalysts showed the higher activities than the conventional alumina-supported catalyst. The catalyst with Al/P atomic ratio of 8 demonstrates the highest HDM activity and the lowest deactivation. The higher the acid amount, the higher the deactivation of the catalyst. The high HDM activities of the CoMo/AAP catalysts can be rationalized on the presence of phosphorus. On the one hand, it can increase the Co-Mo-S groups on the surface and hydrogenation capabilities. On the other hand, it can enhance the acidities and cracking abilities of the catalysts.