Graft copolymers of acrylonitrile (AN)-styrene (ST) onto polybutadiene were prepared via emulsion polymerization method using a redox initiator system, cumene hydroperoxidetetrasodium pyrophosphate-ferrous sulfate-dextrose. The effects of initiator, reaction temperature, reaction time, polymer/grafting monomer ratio, and monomer dropping frequency on the graft copolymerization reaction were investigated. Transmission electron micrographs confirmed that the synthesized particles had a core-shell shaped structure. The powdery core-shell shaped poly(butadiene-g-AN-ST) particles with various AN contents in their shells and with different shell thicknesses were prepared, which can be incorporated into brittle epoxy resin as impact modifier.