A typical fr ee radical grafting system is composed of a polymer substrate, a vinyl bearing monomer and a free radical initiator (usually a peroxide). When the reaction is to be carried out in a co-rotating twin screw extruder there are two main ways to feed the chemicals to the extruder: either they ave fed together to the main hopper (one-pot feeding) or only the polymer is fed to the main hopper and the grafting monomer and the peroxide are injected downstream either just before or after the first kneading zone (sequential feeding). In this study, we investigate the influence of feeding mode on the free radical grafting. We use the free radical grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) as a model system. It is found that GMA＇s grafting yields are very close between the one-pot feeding and the sequential feeding just before the first kneading zone. However those obtained with the sequential feeding just after the first kneading zone are much lower This dramatic difference is closely related to the mixing environments GMA and the peroxide are subjected to. When GMA and the peroxide ave injected before the kneading zone where the screw channel is fully filled, GMA and the peroxide are forced to mb with PI? When they are fed to the extruder just after the kneading zone, however, they simply drip on the highly viscous PP As such, before the grafting takes place between PP and GMA, the peroxide may have started to decompose and GMA may have started to polymerize. It is recommended that low molecular weight ingredients be fed to the extruder through a fully filled zone.