Many researchers have attempted use of nanocomposite (NC) materials for insulation systems at various places. A number of studies on the propagation of electrical trees have been reported, but it has not yet been clarified how the electrical tree occurs inside the NC material. In order to evaluate the origin of an electrical tree, it is important to clarify the breakdown mechanism at nano-scale. In this paper, silica/epoxy-resin NC was spin-coated on a silicon substrate and characterized by thin film analyses. The scanning electron microscope (SEM) indicates uniform dispersion of silica fillers in the NC film. The time-To-breakdown of the NC film, which is measured using micro-electrical probe system, is improved as the silica density increases. Furthermore, we succeeded in the scanning tunneling microscope (STM) observation of the NC film. Leakage sites appeared in the STM images, which were induced by the electric stress application with the STM tip. These approaches will invoke a deep understanding of the role of nano-fillers in the insulation resistance and the breakdown mechanism of NC materials.