Without chemical crosslinking，the graft copolymer EPR-g-PS of ethylene．propylene (EPR) backbone with polystyrene (PS) branches possesses mechanical properties comparable with those of vulcanized EP terpolymers．A tensile strength of 198 kg／cm2．300％ modulus of 113 kg／cm2．and an elongation at break of 705％ were achieved with the sample contain-ing 37.5％ PS (molded speciment)．The tensile strength of specimens (cast film) is higher than that of Kennedy’s EPR-g-PS prepared by cationic grafting reaction．PS-content in the copolymers and average number of branches per molecule are main factors affecting the me-chanical properties of EPR-g-PS．Highly dispersed microdomains，formed by PS branches，act as a thermoplastic network and energy-absorbing“reinforcing filler”，actually undergoing deformation under stress，and are responsible for the thermoplastic elastomeric properties of EPR-g-PS．