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Tree initiation time was evaluated by a pulse detection system to detect a small PD (partial discharge) signal that might take place at first just after a tree was formed. Six kinds of materials were prepared for evaluation, i.e. base epoxy resin, microcomposite, two kinds of nanocomposites, and two kinds of nano-micro-mixed-composites. Lengths of trees detected were around 100 μm in our experiments. As a result, it was clarified that the tree initiation time is prolonged by the nanofiller inclusion, but shortened by the microfiller filling. Furthermore, the co-addition of nanofillers to the microcomposite increases the tree initiation time again in comparison to the once-shortened time in the microcomposite. As for the PD erosion evaluated for the same materials, a good correlation was confirmed between the tree initiation time and PD resistance. This correlation indicates that the tree formation process includes PD erosion. Simulation results show that trees should propagate between fillers.