Abstract: A series of PB-g-SAN elastomers in which the polybutadiene (PB) content is 40% were synthesized using seed emulsion polymerization technique to graft-copolymerize styrene and acrylonitrile onto PB latex rubber particles. The effects of tert-dodecyl mercaptan (TDDM) content in monomer mixture on the grafting behavior of styrene-acrylonitrile (SAN) copolymers and the morphology and dynamic mechanical properties of elastomers were studied. It was found that the grafting degree and efficiency of SAN copolymer decrease and the size of SAN glassy sub-inclusions distributed in PB particles increases but the low temperature dynamic shear modulus of elastomer decreases with the increase of TDDM content. In a series of SAN resins blended with the elastomers particles, the increase in toughness of the blends is found more efficient when introducing TDDM into monomer mixture in the process of PB-g-SAN synthesis.