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In this paper, micro (mean size is 1 μm) and nano (mean size is 10 nm) TiO2 (Titania) were loaded into epoxy resin at the same time. Eleven kinds of samples with different filler contents were prepared. Flashover characteristics of the epoxy composites were investigated under 50 ns (rise time)/1.5 μs (half-height width time) unipolar pulse and 5 × 10-3 Pa vacuum degree. It was found that the nano TiO2/epoxy composites are of higher flashover voltage than neat epoxy, and the addition of micro TiO2 in appropriate amount was able to further improve the flashover strength of the nano TiO2 /epoxy composite. In order to research the mechanism of micro and nano fillers, the permittivity and thermally stimulated depolarization current of the samples were carried out. It was clarified that, when micro filler is added into nano composite, deeplike traps can be increased remarkably, whereas shallowlike traps increase much less and even decrease in some samples. Considering the result of flashover voltage, it was confirmed that deeplike traps will increase flashover voltage.