Oxidative desulfurization (ODS) of diesel fuels with hydrogen peroxide was studied, using activated carbon as the catalyst. The adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT), and the correlation between them, were investigated. The higher the adsorption capacity of the activated carbons, the higher the catalytic performance in the oxidation of DBT. DBT is oxidized by active oxygen species on the carbon surfaces when DBT is adsorbed on the carbon surfaces. The effect of the aqueous pH on the catalytic activities of the activated carbons was also investigated. It is observed that the oxidation rate of DBT on the carbons is enhanced when the aqueous pH is <2. The addition of formic acid can promote the oxidation of DBT in the presence of the wood carbons. The ODS of a commercial diesel fuel (with a sulfur content of 800 ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. A much lower residual sulfur content (142 ppm) in the oxidized oil is observed after the oxidation, using the hydrogen peroxide−activated carbon−formic acid system, compared to that for a hydrogen peroxide−formic acid system, and the resulting oil, which contained 16 ppm of sulfur, is obtained after activated carbon adsorption; 98% of the sulfur could be removed from the diesel oil, with 96.5% oil recovery. Activated carbon has a high catalytic activity for the oxidative treatment of diesel fuels and can be used repeatedly. The reaction and adsorption in the desulfurization process are coupled, which causes low residual sulfur content in the oxidized oil. These results may indicate that activated carbon has a potential application in ODS to meet future regulations of sulfur in diesel fuels.