Volatile organic compounds (VOCs) can be produced from a variety of industrial and commercial processes, including printing, metal decorating, paint drying, manufacturing of organic compounds and polymers. Granular activated carbon (GAC) is widely used to remove VOCs from gas streams. In this study, coconut shell carbons were chemically treated by ammonia, sodium hydroxide, nitric acid, sulfuric acid and phosphoric acid. The mechanisms of o-xylene, one of the major VOCs, adsorption by modified GAC were systematically studied. The saturated adsorption capacity of o-xylene on the GAC was measured by the gravity method and the adsorption effect of the original and modified activated carbons were compared. Results showed that the GAC modified by alkalis had the better adsorption capacity of o-xylen. SEM/EDX and BET were used to identify the structural characteristics of the original and modified GAC. The surface functional groups were analysed by the IR spectroscopic method and Boehm’s titration technique. Surface acidity of GAC was determined by electrophoretic mobility measurements. The relationships between physicochemical characteristics of GAC and their adsorption performances demonstrated that the adsorption capacity of o-xylene was related with the microspore surface area and functional groups of the surface of GAC. Surface functional groups of carboxyl and phenolic hydroxy increased upon ammonia and sodium hydroxide treatment.