Cross-linked a-methylstyrene and acrylonitrile (MStAN) copolymer particles in a latex form were synthesized by free radical emulsion polymerization. The particles showed a narrow size distribution and an average diameter of 65 nm. The amount of the vinyl groups at the surface of the (MStAN) copolymer particles resulted from varied amount of the initiator (APS) and the crosslinking agent (DVB) was detected by iodine titration method. When filled into ethylene propylene rubber (EPR), the MStAN nanoparticles exhibited excellent reinforcing capabilities, and with the increase of the vinyl groups at the surface of MStAN particles, the tensile stress of MStAN/EPR increased. Results demonstrated that the vinyl groups at the surface of the (MStAN) copolymer particles provided certain chemical interactions between the filler particles and the macromolecular chains of rubber matrix. V C 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41914.