Abstract: Hydrodesulfurization(HDS) of 4-methyldibenzothiophene(4-MDBT) and dibenzothiophene(DBT) on sulfided CoMo/γ-Al2O3 catalyst was studied. Moreover, reaction networks and mechanisms were revealed on the basis of GC and GC-MS analyses of the reaction products. It was found that the HDS of 4-MDBT on sulfided CoMo/γ-Al2O3 catalyst occurs through the direct hydrogenolysis of C-S bond of 4-MDBT and hydrogenation routes, the con-tributions of both routes being comparable. HDS of DBT on the sulfided CoMo/γ-Al2O3 catalyst occurs essentially through the direct hydrogenolysis route. The methyl group in 4-MDBT causes the spatial restraining for the “end up adsorption” of sulfur atom on the active site of the catalyst, which slows down the reaction rate of the direct hydro-genolysis route and leads to the reduction of conversion of 4-MDBT. The reaction rate of the hydrogenation routes in HDS of 4-MDBT and DBT obviously decreases with the descending of reaction pressure. Reaction pressure was also found to have smaller and opposite effects on hydrogenation routes and the direct hydrogenalysis routes in HDS of 4-MDBT and DBT, i.e., the reaction rate of the direct hydrogenalysis routes slightly ascends with the descending of reaction pressure. The effect of reaction pressure on the conversion of 4-MDBT is higher than that on DBT. Reaction temperature has obvious effect on both the direct hydrogenolysis routes and the hydrogenation routes in HDS of 4-MDBT and DBT. Moreover, effect of temperature on the direct hydrogenolysis route in HDS of DBT is lower than the hydrogenation route. However, the effect of temperature on the direct hydrogenolysis route in HDS of 4-MDBT is slightly higher than the hydrogenation route. The electron donor induction of the methyl group in 4-MDBT can promote the hydrogenation of the adjacent benzene ring.