Graphite nanoplatelets (GNPs) are performed to fabricate GNPs/bisphenol-A epoxy resin (GNPs/E-51) nanocomposites with high thermal conductivity via casting method. And the "two-step" method of methanesulfonic acid/gamma-glycidoxypropyltrimethoxysilane (MSA/KH-560) is introduced to functionalize the surface of GNPs (fGNPs). The KH-560 molecules have been successfully grafted onto the surface of GNPs. The thermal conductivities of the fGNPs/E-51 nanocomposites are increased with the increasing addition of fGNPs, and the corresponding thermally conductive coefficient of the fGNP5/E-51 nanocomposites is improved to 1.698 W/mK with 30 wt% fGNPs, 8 times higher than that of original E-51 matrix. The flexural strength and impact strength of the fGNP5/E-51 nanocomposites are optimal with 0.5 wt% fGNPs. The thermal stabilities of the fGNP5/E-51 nanocomposites are also increased with the increasing addition of fGNPs. For a given GNPs loading, the surface functionalization of GNPs by MSA/KH-560 exhibits a positive effect on the thermal conductivities and mechanical properties of the nanocomposites. (C) 2015 Elsevier Ltd. All rights reserved.