A series of CoMo/Al2O3 catalysts was prepared using different crystal forms of alumina (including gamma-Al2PO3, delta-Al2O3, theta-Al2O3, and alpha-Al2O3) as supports for the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of fluid catalytic cracking diesel. The physicochemical properties of the supports and the corresponding catalyst were analyzed by various characterization methods. The results showed that delta-Al2O3 possessed a moderate surface area, a concentrated pore size distribution, and less surface hydroxyl groups. The CoMo/delta-Al2O3 catalyst exhibited the highest HDS and HDN efficiencies, 98.4 and 96.1%, respectively. This could be attributed to its reduced metal-support interaction, moderate stacking morphology, and highest sulfidation degree of active phases. The HDS and HDN efficiencies of the catalysts followed the order of CoMo/alpha-Al2O3 (87.3 and 72.2%) < CoMo/gamma-Al2O3 (94.8 and 87.9%) < CoMo/theta-Al2O3 (96.2 and 90.1%) < CoMo/delta-Al2O3 (98.4 and 96.1%).