Crosslinked alpha-methylstyrene and acrylonitrile (MStAN) copolymer particles in a latex form were synthesized by free radical emulsion polymerization. The particles took a spherical shape with an average size of 53.1 nm in a narrow distribution. When filled into styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), and natural rubber (NR), the MStAN nanoparticles exhibited excellent reinforcing capabilities and the best in NBR. By the employment of heat treatment, mechanical properties of the MStAN-filled SBR composites had got remarkable further improvements. But mechanical properties, together with the morphology, of the MStAN-filled NBR composites, varied little after heat treatment, which, however, divulged the naturally good compatibility between the MStAN particles and the NBR matrix. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2459-2467, 2011