Graphene nanosheets-poly(methyl methacrylate) (GNS-PMMA) nanocomposites were first prepared by in situ suspension polymerization and reduction of graphene oxide using hydrazine hydrate and ammonia. PMMA microspheres with a mean diameter of 2 mu m are mainly covalently link to the surface of GNS. The obtained GNS-PMMA composites have not only high electrical conductivity but also enhanced mechanical properties and thermal stability at low loadings of graphene. Especially, the resulting nanocomposites were examined for electrorheological fluids, showing thin and dense chains of particles after application of an electric field. (C) 2011Wiley Periodicals, Inc. J Appl Polym Sci 122: 1866-1871, 2011