Core–shell polybutadiene-graft-polystyrene rubber particles with different ratios of polybutadiene core to polystyrene shell were synthesized by an emulsion polymerization using K2S2O8 as an initiator. Then the core–shell rubber particles were blended with PS to prepare PS/PB-g-PS. The rubber particles with a size of 0.3–0.5 mm could toughen polystyrene significantly. The mechanical properties, morphologies and deformation mechanisms of samples were extensively investigated. The experimental results showed that the dispersion of rubber particles in a ‘‘cluster’’ state leads to better impact resistances. Crazing occurred from rubber particles and extended in a bridge-like manner to neighboring rubber particles parallel to the equatorial direction.