Dibenzothiophene sulfur (DBTs), the most stable sulfur species, is present in remarkable concentration in petroleum. Removal of DBTs is with profound significance in environmental protection. In this work, a density functional theory method was adopted to investigate the pyrolysis mechanism of DBT. It was found that the pyrolysis of DBT is possibly started by H-migration or S-C bond rupture. Three main reaction pathways were found. Two dominating pyrolysis pathways are through thiol intermediate pyrolysis and another is through DBT carbene direct dissociation. The dominating products are sulfur-free atoms, 2-ethynyl-benzothiophene, 3-ethynyl-benzothiophene, ethyne, biphenylene, and 1,8-dihydrocyclopentaindene, with a certain amount of benzothiophene and SH radicals. For DBT, BT, and thiophene, the most difficult step is the initial step. The highest energy barrier of DBT is about 17 kcal/mol higher than BT and thiophene, indicating that pyrolysis of DBT needs more intense reaction conditions. Thiophenes can be stable in extreme conditions, which is possibly caused by the high energy barrier of the initial reaction step. After the initial reaction steps, BT and DBT can be desulfurized more easily through thiol intermediates. Searching the catalyst that could lower the energy barrier of the initial step and the reagent that could convert thiophenes directly into thiols may be a potential desulfurization approach for thiophenes.