The reactive extrusion process of isotactic polypropylene (iPP) grafting maleic anhydride (PP-g-MAH) initiated by dicumyl peroxide (DCP) in the presence of supercritical carbon dioxide (scCO(2)) is investigated. Because of its moderate supercritical conditions and well swollen performance in iPP melts, scCO(2) is selected to be imported into the extruder system to reduce process temperature and melt viscosity as well as strengthen the mass transport. It has been found that the process temperature of reactive sections of co-rotating twin screw extruder can be successfully reduced from conventional 190 to 160 degrees C when assisted with the addition of scCO(2). Consequently, effective suppression of main chain degradation is observed, which leads to the products with relative higher molecular weight and narrower molecular weight distribution. The experimental results also indicate a significant increase in both the grafting degree of resultant PP-g-MAH and the grafting efficiency of MAH under certain operation conditions. Especially, the grafting efficiency is close to 90% when low concentration of both MAH and DCP are employed. A potential mechanism has been proposed to explain the effects of scCO(2) in the reactive extrusion. In comparison with traditional molten grafting process, the work presents a novel approach to increase the grafting efficiency of MAH and control the molecular weight of resultant PP-g-MAH simultaneously. (C) 2009 Elsevier Ltd. All rights reserved.