The graft copolymerization of styrene (St) onto random ethylene-propylene diene monomer (EPDM) with benzoyl peroxide (BPO) and 2,2-azo-bis-isobutyronitrile (AIBN) as the initiator in solvent was studied. Results showed that BPO was superior to AIBN. The influence factors, such as reaction St and BPO, on the extent of graft were discussed. The existence of a grafted copolymer was verified by infrared spectra, NMR, and thin-layer chromatography. The grafted copolymer was characterized by thermally stimulated current.