This study describes the emulsion grafting of styrene and acrylonitrile onto 60-70% polybutadiene (PB), in the presence or absence of tert-dodecanetiol as a chain transfer reagent with a radical initiator, and the properties of the obtained grafted stock. There was no significant difference in terms of effect of the initiation mode on the grafting efficiency resulting from the high grafting reactivity of PB, However, the grafted stock with 70% PB prepared in the presence of tert-dodecanetiol and the adequate selection of an initiation system gave a homogeneous dispersion of the PB particles into poly(styrene-co-acrylonitrile) (SAN) matrix. The initiation system involves tert-butyl peroxylaurate, tert-butyl peroxyacetate, and tert-butyl peroxyisopropylcarbonate coupled with ferrous sulfate. The efficient coverage of the SAN grafted layer around 70% PB particles was observed by TEM to eventually give excellent impact resistance, high surface gloss, and good thermal resistance. The absence of tert-dodecanetiol resulted in a toughness reduction of ABS.