In this study, the unsaturated polyester resin/graphite nanosheet conducting composites with a low percolation threshold of 0.64 vol% have been prepared via in situ polymerization under the application of ultrasonic irradiation. Two theoretical approaches (mean-field theory and excluded volume theory) are applied to interpret this low critical volume fraction and the latter can explain the low value better. The microstructures reveal that low value of percolation threshold may be mainly attributed to better conductive network consisting of graphite nanosheet with special morphology and there exists contact resistance in the percolating network formed within unsaturated polyester resin/graphite nanosheet composites. Furthermore, preliminary studies on the influence of graphite nanosheet on the thermal stability of the host unsaturated polyester resin have been performed. (c) 2006 Elsevier Ltd. All rights reserved.