Enameled wire is composed of conductive core surrounded by insulating layer. Nanocomposite enameled wires where inorganic nanoparticles are dispersed in the insulation layer have been known to exhibit higher breakdown voltage and partial discharge resistance. Dispersion of the nanoparticles is critical for fabricating nanocomposite enameled wire because nanoparticles easily aggregate in the varnish or polymer matrix during the coating processing. In this study silica nanoparticles were surface-modified with silanes using sonochemical method. The surface modified nanoparticle was well dispersed in polyester varnish, which was observed by means of optical transparency and viscosity measurement. After curing, it was found that dispersion of the nanoparticles was also well maintained, which was confirmed by SEM and DSC analyses. Consecutively we found that thermal resistance as well as dielectric breakdown strength of the nanocomposite enameled wire was enhanced by dispersing the surface modified silica nanoparticles in the insulating polymer matrix. (C) 2011 Elsevier B.V. All rights reserved.