The graft copolymerization of styrene and methyl methacrylate on SBR latex particles in the core-shell emulsion process was conducted in a 600 mL glass stirred vessel with the BPO-Fe2+ redox initiator. The effects of the principal factors such as the polymerization temperature, monomer-to-polymer ratio, the frequency of monomer addition and conversion on the grafting degree, and the grafting efficiency were studied. The surface-controlled process model was proposed to describe the grafting mechanism.