Abstract: Dibenzothiophene (DBT) and its homologues are important molecular markers in petroleum geochemistry. However, DBT-related geochemical indicators were derived mainly based on empirical observations due to insufficient understanding of the mechanisms through which these organosulfur compounds were generated in sedimentary environments. Thermal simulation experiments with 3,3’-dimethylbiphenyl and sulfur in closed gold tubes at 200-500 ℃ and 10 MPa afforded three main dimethyldibenzothiophene isomers (DMDBTs). Importantly, DBTs could be generated at a temperature as moderate as 200 ℃ under geological conditions, because the temperature threshold of the reaction could be reduced by increasing the system pressure. Further investigations revealed that the regioselectivity of the thiophene formation reaction was strongly influenced by temperature. This is similar to the process that occurs in natural geological systems. It should also be noted that the maturity index 4-/1-MDBT (MDR) reaches a maximum around 1.5% (Easy%R_o). It indicates that MDR index should be used in a suitable maturity regions combined with the given geological setting.