Nanocomposites have been known in the field of electrical engineering for more than a decade, albeit there are still uncertainties of how the filler properties influence the properties of the composite material. Filler size, shape, aspect ratio and the surface modification, which consequently leads to even dispersion or clusters of particles, are all suspected to influence the dielectric behaviour of the insulation. But it is disputed what influences the properties to what extent. The focus of this paper is the dependence of the short term breakdown (BD) strength of polymer based composites filled with boron nitride (BN) on the filler size. Base polymer is commercially available bisphenol A epoxy with anhydrite hardener. As filler material we use boron nitride powder with different average particle sizes. These are ranging from 70 to 5000 nm. The particles were not surface modified in order to see solely the influence of the filler size on the BD strength. The short term BD strength was measured for negative DC voltages with Rogowski shaped electrodes. Dielectric spectroscopy was done complementarily to see how the filler size influences the relative permittivity of the composites.@en