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Epoxy resin is one of the most widely used thermosetting polymers and commonly applied in electrical power engineering. The intrinsic properties of epoxy can be improved by the introduction of inorganic filler, thus fabricating a composite material. In this study we investigate the influence of the filler size and its concentration on the complex permittivity and thermal conductivity of epoxy. The filler material that has been used is boron nitride. Boron nitride is a ceramic dielectric material with high electric breakdown strength and high thermal conductivity, while exhibiting a low relative permittivity. Four types of composites were prepared with an average particle size of 70 nm, 0.5 μm, 1.5 μm and 5 μm and a filler concentration of 10 and 20 percent by weight. The surface of the particles was functionalized with a coupling agent in order to see the effect of the modification on the specific properties. Two different techniques were applied to get the particles well dispersed in the matrix: high shear mechanical mixing and ultrasonication. The results of dielectric spectroscopy show, that the relative permittivity of the composite material increases more when introducing particles which have a larger average size. The highest value of thermal conductivity was obtained for the composite containing 20 wt.% inorganic particles with average particle size of 0.5 μm. Explanations are given for the observed behaviour.