Diels-Alder reaction is a useful method for modifying the surface of carbon materials. In this work, we functionalized activated carbon with a large number of carboxyl groups using the Diels-Alder reaction. Polyaromatic structure of activated carbon acted as a diene for addition of maleic anhydride (MA) that acted as a dienophile. Upon hydrolysis of the anhydride group in the adduct, vicinal carboxyl groups were formed. The formation of carboxyl groups by Diels-Alder reaction was proved by FTIR, solid-state C-13 NMR, CO and CO2 evolution from TPD and Boehm titration. Titration results showed that the number of carboxyl groups increased from 0.09 mmol g(-1) to 3.06 mmol g(-1). DFT calculation showed that the concave site of the armchair edge on carbon surface was the most favorable for the Diels-Alder addition. The formation of exo-adduct was preferred with a reaction energy (Delta E) of -24 kJ mol(-1). The synthesized catalyst was tested for acid-catalyzed hydrolysis of cellobiose. The catalyst showed higher activity than a catalyst prepared by oxidation in the presence of air.