Abstract: Efforts have been made to use the graft copolymer EPR-g-PS with ethylene and propylene copolymer(EPR) as the backbone and polystyrene(PS) as grafts, prepared by macromer copolymerization, as a compatibilizer for PS/EPDM blends. The results indicated that composition of the blends, dosage and molecular structure of the graft copolymers had strong effects on impact strength of the blends. The mechanism of the graft copolymers as a compatibilizer for PS/EPDM blends has been interpreted by correlating the structure of the graft copolymers with the morphology of the blends revealed by phase contrast microscopy and scanning electron microscopy.