Abstract: A carboxyl-functionalized ionic liquid, 1-methyl-3-methylcarboxylate imidazolium tetrafluoroborate (cmmimBF₄), was applied as the catalyst and extractant in the oxidative desulfurization system for the removal of dibenzothiophene (DBT) in the presence of H₂O₂ in model oil. Several parameters including the amount of hydrogen peroxide, temperature and reaction time were investigated. The catalyst used shows a good activity with a DBT conversion of 100% at 60 ℃, O/S mol ratio 6 and the reaction time of 80 min. The activity of ionic liquid BF₄ decreases little after recycling for 4 times.