Submicrometer core-shell polybutadiene-graft-polystyrene (PB-g-PS) copolymers were synthesized by emulsion grafting polymerization with 1,2-azobisisobutyronitrile as an initiator. Then, PB-g-PS copolymers were blended with high-impact polystyrene (HIPS) and PS to prepare HIPS/PS/PB-g-PS. It is generally accepted that PS could only be toughened by 1–3-lm rubber particles effectively. From the Izod impact test, however, the results showed that PS could be toughened effectively by submicrometer rubber particles(around 300 nm) in our study. Moreover, the impact strength of HIPS/PS/PB-g-PS is much higher than that of HIPS due to the synergism between submicrometer rubber particles and micrometer salami particles. Subsequently, the stress-whitening zones for HIPS, PS/PB-g-PS, and HIPS/PS/PB-g-PS were also observed in detail by transmission electron microscope. Based on mechanical properties and deformation morphologies, the deformation mechanism in the HIPS/PS/PB-g-PS blends was given as follows: crazes could be initiated from large rubber particles, passed through ‘‘craze-bridge’’ rubber particles, and ended to neighboring small one, which prevented excessively crazing to be form cracks, absorbed energy, and finally enhanced the toughness of PS.