A series of submicrometer-sized polybutadiene (PB) rubber particles with different diameters ranging from 100 to 450 nm were prepared via an emulsion polymerization. Subsequently, core.shell polybutadiene-graf t-polystyrene (PB-g-PS) particles were synthesized using a redox initiator system (cumene hydroperoxide and ferrous sulfate) by an emulsion grafting polymerization. Then the submicrometer sized PB-g-PS impact modifiers were utilized to toughen polystyrene (PS) by blending PS and high-impact polystyrene. From measuring mechanical properties, it was found that the notched impact strength of toughened PS was significantly influenced by core.shell PB-g-PS particle size. When the rubber particle size was above 300 nm, the notched impact strength was 208 J/m, nearly 2 times than that of toughened PS with particle size 100 nm. Moreover, the stress whitening zones were observed by transmission electron microscopy, and the result showed that multiple crazing is its dominative toughening mechanism.