In the present work the influence of nano-scaled zirconia particles with different structures on the structural and electrical properties of polyurethane (PU) insulating coatings was investigated. PU matrix insulating coatings containing non-doped and 4 mole % yttrium doped nano-scaled zirconia with average particle size between 12 and 13 nm were prepared by wet chemical methods. High resolution transmission electron microscopy analysis of thermally cured composites showed that the nano dispersed zirconia particles were uniformly distributed within matrix. Incorporation of 5 vol. % nanoparticles into the PU matrix resulted in a slight increase of the micro hardness and elastic modulus. The effect of zirconia filling (non-doped and doped) on the electrical breakdown strength was not observed. But, a strong influence of the nanoparticle addition on the partial discharge stability was detected. Partial discharge breakdown time for non-filled PU matrix was 2.5 h, for PU filled with non-doped zirconia 65 h and for PU filled with yttrium doped zirconia >; 100 h, respectively. The additional benefit of doped zirconia may be explained by its different electronic structure and polarisation behaviour.