Nanocomposite insulation materials composed of polymer and nano-fillers have been at the cutting edge of material technology. They will enable high efficiency and downsizing of electrical rotating machines. Profound understanding of the nano-filler effects in epoxy-based micaceous insulation systems is essential before they can be applied to electrical rotating machines. In this paper, therefore, we newly develop the evaluation method using the needle-shaped electrode 0.1 mm in thickness. The electrode embedded micaceous samples impregnated with nano-filled epoxy have longer insulation breakdown time than that with neat epoxy. Besides, the samples for observation with thin electrode between two mica tapes enable us to compare the behavior of electrical tree propagation in the interlayers impregnated with nano-filled and neat epoxy.