Very thin graphite nanosheets (GNSs) with 11 nm thickness and 250 nm in lateral size were simply prepared from bulk graphite via an ultrasonication and centrifugation in N,N-dimethylformamide (DMF) without any chemical treatments such as using surfactants. GNSs/epoxy nanocomposites with 3 wt% GNSs showed a high dielectric constant (710) and relatively low dielectric loss tangent (less than 0.5) at low frequency (100 Hz). This great improvement in the dielectric properties could be attributed to very thin thickness, excellent dispersibility and sheet-like shape of GNSs in comparison with graphite or thick GNSs.