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Epoxy resin is widely used as an insulation material in many electrical apparatuses because of its excellent electrical and manufacture characteristics. It is usually mixed with filler to improve mechanical and thermal characteristics. In order to qualitatively clarify the effect of fillers on treeing phenomena, treeing tests were performed with epoxy specimens mixed with different kinds of fillers. The property of the interface is also clarified by the treatment of silica filler using silane coupling. Although tree initiation voltage decreased with introducing silica fillers, the fillers prevented the growth of the tree. The effect of filler shape was more significant on round-shape filler than on square-shape filler. Silane coupling treatment to the fillers did not bring a significant change in tree initiation voltage, however, brought a reduction in tree propagation. The change in tree propagation rate was explained by considering the field relaxation and energy dispersion due to branching at the filler-resin interface. Tree propagation along the interface between resin and a quartz plate was observed and analyzed in order to ensure the above explanation.