Co _ Mo/Al2O3 catalysts with high loading of Mo (20 wt% Mo) were prepared by the addition of citric acid (CA) using a simultaneous impregnation method or a post-treatment method. The catalyst activity was investigated for the hydrodesulfurization (HDS) of dibenzothiophene. TEM, Co K-edge XANES, NO adsorption capacity, and surface area measurements were conducted to characterize the catalysts. The catalysts were also investigated by the chemical vapor deposition technique using Co(CO)3NO to prepare Co/MoS2 and Co/Co _ MoS2 catalysts. Addition of citric acid by the post-treatment method to calcined (Co) _ Mo/Al2O3 oxide catalysts significantly enhanced the edge dispersion of MoS2 particles and the catalytic activity of Co/Mo/Al2O3 and Co _ Mo/Al2O3, whereas citric acid addition by the simultaneous impregnation method hardly affected the edge dispersion. The highest HDS activity was obtained with a CA/Mo mole ratio of 1.5-2 for the post-treatment catalysts. The posttreatment method may be applicable to the regeneration of deactivated HDS catalysts.