Graphene oxide (GO) and its derivatives with vinyl and alkyl functional groups (mGO) were synthesized and dispersed into unsaturated polyester resin (UPR) to prepare nanocomposites. Successful chemical modification of the GO sheets was confirmed by infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction. In contrast to unmodified GO, mGO was easily dispersed in UPR, even without sonication, making the process scalable. The few-micrometer GO sheets and aggregates in the resin composite were characterized by transmission electron microscopy and visible light microscopy. Compact tension testing of the resin composites with mGO showed that, with a remarkably low loading of only 0.04 wt% mGO, a 55% improvement in fracture energy (G(IC)) was obtained with little change in flexural strength or modulus. This high effectiveness renders mGO economically viable. Analysis of fracture surfaces by scanning electron microscopy suggests that mGO particles interact with the propagating crack, the main toughening effect being crack pinning. (C) 2017 Elsevier Ltd. All rights reserved.