The grafting reaction of maleic anhydride to molten iPP was investigated in both an internal mixer and a twin- screw extruder The influence of peroxide content, added maleic anhydride and process variables such as residence time, screw speed and reaction temperature were considered. The rate and level of grafting were quantitatively measured by FTIR and titrometery technics. Maximum level of grafting at constant peroxide content was obtained when concentration of added maleic anhydride reached up to 6%, above which is declined. The rate of grafting increased with increasing the peroxide content. However the beta-scission of polypropylene was found to be taken place as a parallel competitive reaction which was intensified with increasing peroxide content. It was demonstrated that it is possible to make an optimization on the grafting reaction on the basis of experimental results in conjunction with a free radical kinetic model developed for this reaction.