The grafting of a vinyl monomer, methyl methacrylate (MMA) or styrene (ST), onto a natural rubber (NR) exhibited a core-shell structure, with NR as the core and poly(methyl methacrylate) or polystyrene as the shell. The grafting efficiency (GE) of the graft copolymer was determined by a solvent-extraction technique. The synthesized graft copolymer was purified and then characterized by Fourier transform infrared spectroscopy analysis. The effects of the graft parameters, including time, initiator content, and concentration, and the type of monomer, MMA or ST, were investigated. A longer time was favorable for the graft copolymerization. GE first increased and then decreased with increasing concentration of initiator. GE decreased with increasing monomer content, and it was confirmed that the graft copolymerization was a surface-controlled process. The grafting ST monomer had a higher GE compared to MMA under the same conditions. The characterization of the particles by transmission electron microscopy and scanning electron microscopy confirmed the formation of a core-shell structure. From the micrographs, we inferred that at 71% GE, the NR seed particle had a complete closed shell several nanometers in thickness. Therefore, the NR particle was dispersed in a polymer nanomatrix. (C) 2008 Wiley Periodicals, Inc.