Polyisoprene-block-polystyrene-block-polyisoprene (ISI) was synthesized by the iniferter route and its use, as compared to a commercial polysoprene-blockpolyisoprene-block-polystyrene (SIS), in the enhancement of the toughness of high-impact polystyrene (HIPS), prepared by the gamma-radiation vulcanized natural rubber (RVNR) latex/phase transfer/bulk polymerization technique, was investigated. Addition of 5% SIS was adequate as an interfacial agent, which effectively increased the unnotched Izod impact energy of HIPS, whereas use of 10% of ISI was required. A long polyisoprene block with two polystyrene segments of SIS was favorable for compatibilization of HIPS. Transmission electron micrographs revealed the uniform distribution of the block copolymer at the shell region of the rubber particle.