Graphene platelets (GP) are a novel class of nanofillers due to its good compatibility with most polymers, high aspect ratio, high absolute strength and cost-effectiveness. We in this study synthesised two types of epoxy/GP nanocomposites with different interface strength using the combination of sonication and chemical modification. Although the surface-modified graphene platelets (m-GP) formed clusters, a higher degree of dispersion and exfoliation of graphene was observed in each cluster owning to the improved interface by modification. The scrolling of graphene was found predominantly in the interface-modified nanocomposite. At 4 wt%, the modified nanocomposite shows fracture energy release rate G(IC) 613.4 J m(-2), while the unmodified nanocomposite indicates 417.3 J m(-2), in comparison with neat epoxy G(IC) 204.2 J m(-2). The interface modification enhanced the glass transition temperature of neat epoxy from 94.7 to 108.6 degrees C, 14.7% increment. Toughening mechanisms are attributed to the voiding, microcracking and breakage of GP, while matrix may not consume as much fracture energy as m-GP do. (C) 2011 Elsevier Ltd. All rights reserved.