A series of supported sulfided Co-Mo/Υ-Al₂O₃ catalysts of different cobalt contents were tested in a fixed bed flow reactor for the hydrodesulfurization of dibenzothiophene(DBT)dissolved in n-heptane. Reaction temperatures were between 533K and 537K, and total reaction pressures were from 20 to 60×10⁵Pa. Most of the products observed were biphenyl and cyclohexylbenzene, and only negligible amount of bicyclohexyl and tetrahydrodibenzothiophene were detected. The catalytic activities with different cobalt contents were shown to be maximum in the range of R=Co/(Co+Mo)=0.2-0.3 under standard condition. The reaction was first order with respect to DBT concentration, and the average activation energy was 23.4kcal/gmole.