Abstract: Hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) on unsupported NiMoW catalyst was studied. Moreover, mechanisms and reaction networks were revealed on the basis of GC and GC-MS analyses of the reaction products. The result shows that, the HDS rate of dibenzothiophenes is in the order of 4,6-DMDBT≈4-MDBT＜DBT for the spatial restraining effect of the methyl group. Unsupported NiMoW catalyst has high hydrogenation activity in aromatics saturation, it favours the hydrogenation of alkyl-substituted aromatic ring, weakens the spatial restraining effect, leads to the effectively removing of alkyl-substituted dibenzothiophenes. The hydrodesulfurization products of DBT could be hydrogenated further and its distribution is similar to the hydrogenation products of biphenyl(BP). The hydrogenation route of the hydrodesulfurization of 4-MDBT have two reaction pathways, the electron donor induction of the methyl group could promote the hydrogenation of the adjacent phenyl. On the lewis acid sites of catalyst, part of 4-MDBT and 4,6-DMDBT transformed into DBT through the demethylation reaction in their hydrodesulfurization process.