The core-shell graft copolymer of alpha-methyl styrene with acrylonitrile on poly(butyl acrylate) was synthesized. The graft polymerization was investigated as a function of reaction temperature, initiator concentration used in the secondary polymerization, monomer to polymer ratio and emulsifier concentration. The compatibility of this core-shell graft copolymer with poly(vinyl chloride) (PVC) was determined by the solubility parameter method and scanning electron microscopy. The mechanical and theological behaviour of the blend show that this core-shell graft copolymer can be used as a processing aid for PVC.