Ethylene-propylene-diene terpolymer (EPDM) was functionalized with well-defined polystyrene (PS) using a reversible addition-fragmentation chain transfer (RAFT) technique, yielding EPDM-graft-PS. PS was first synthesized using the RAFT technique employing trithiocarbonates as a chain transfer agent. PS was then grafted to EPDM in the presence of a radical generator that endows reactive radical sites on EPDM. The resulting EPDM macro-radicals were chain transferred to the trithiocarbonate units in the PS. The reaction and structure of the resulting graft copolymers were elucidated by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). The composition and grafting efficiencies were also adjusted by controlling the amount of radical generator and PS.