Polystyrene-block-poly(glycidyl methacrylate) reactive diblock copolymer (PS-b-PGMA) was synthesized via atom transfer radical polymerization (ATRP). The diblock copolymer was characterized using nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The cured epoxy thermosets with 10-20 nm PS particles were prepared by blending the diblock copolymer with epoxy resin. The nanostructures were examined by means of transmission electronic microscopy (TEM) and small angle X-ray scattering (SAXS). The formation of the nanostructures was caused by the reaction-induced microphase separation mechanism. It is significant that the glass transition temperatures (T(g)s) of these epoxy thermosets were increased by the addition of PS-b-PGMA reactive block copolymer as revealed by both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 2110-2118,2010