A nanofluid is a two-phase mixture composed of a liquid phase and solid nanoparticles in suspension. These nanoparticles exhibit unique properties, compared to those of the same material at bulk scale. In this study, mineral oil based fullerene nanofluids with different concentrations were prepared. Of main interest was the AC breakdown voltage of the nanofluids. The AC breakdown voltage of the nanofluids was tested at around 24ppm humidity level. Since viscosity has such a large impart on properties of liquids, viscosity tests of fullerene nanofluids were performed with a rheology meter, in order to see if observed changes in the AC breakdown strength could be related to a change of viscosity.