The graft copolymerizations of vinyltrimethoxysilane (VTMO) and styrene (St) onto ethylene-propylene-diene terpolymer(EPDM) were carried out with benzoyl peroxide (BPO) as an initiator in toluene. The effects of EPDM concentration, mole ratio of VTMO to St, reaction time, reaction temperature, and initiator concentration on the graft copolymerizations were examined. The synthesized VTMO-EPDM-St graft terpolymers (VES) were confirmed by infrared and H-1-NMR spectroscopies. The molecular weight, thermal stability, light resistance, and weatherability of the graft terpolymer were investigated by gel permeation chromatography, thermogravimetric analysis, and Fade-o-Meter. The number-average molecular weight was 109,000. It was found that the heat resistance and light resistance as well as weatherability of VES are considerably better than those of acrylonitrile-butadiene-styrene terpolymer.