Very thin graphite nanosheets are obtained using an ultrasonic irradiation method, and epoxy/graphite nanosheet composites with different filler content are fabricated using the diglycidylether of bisphenol A epoxy matrix. An investigation of structural characteristics and mechanical and dielectric properties of the nanocomposites is carried out. SEM micrographs shows that the thickness of a single layer graphite nanosheet is about 20 nm whereas FTIR studies indicates that the surfaces of the graphite nanosheets are enriched with hydroxyl and carbonyl groups. The dielectric constants of the composites are increased with increases in graphite nanosheet content lower than 3.5 wt%, and are still higher than 100 in the high frequency range with 3.5 wt% graphite filler content. The tensile strength and storage modulus of the composites increase with increasing nanosheet concentration. These epoxy/graphite nanosheet composites, which show both high dielectric constant and toughness, could have potential application in embedded capacitor technology. POLYM. ENG. SCI., 50:1734-1742, 2010. (C) 2010 Society of Plastics Engineers