If a power semiconductor module becomes compact to enhance its wiring density, the electric field at its insulation part should become high. This in turn causes the increase in heat generation per unit volume. Therefore, epoxy resin with high thermal conductivity, a low thermal expansion rate and good anti-aging performance is required. In this regard, we made nano-micro composites (NMCs), namely epoxy resin samples containing both nano-silica filler having high insulating performance and micro-alumina filler having a high thermal conductivity, and observed electroluminescence (EL) and dissipation current waveforms under AC high electric fields for these NMCs simultaneously in order to examine the effects of addition of fillers with different sizes. As a result, it was found that both dissipation current and the number of EL pulses depend on the sample thickness. It was also found that the addition of alumina-micro filler alone into epoxy resin increases the number of EL pulses compared to the neat epoxy and that the NMC shows improved insulating properties. It was also confirmed that the relative permittivity is around 4 for the neat resin and the NMCs, independent of the thickness and the type of fillers. © 2013 IEEE.