Effects of nanofiller addition on four typical dielectric properties, namely permittivity εr',dielectric loss factor εr", space charge accumulation, and partial discharge (PD) resistance were evaluated for polypropylene (PP) and its nanocomposites (NCs) with nanoclay. While εr' and εr" are almost independent of temperature and frequency in the base unfilled PP, they are highly dependent on the two parameters in the two NCs. Namely, εr' increases significantly at temperatures above 20 °C and the frequency spectrum of εr" shows at least one temperature-dependent peak. Furthermore, space charge appears abundantly in the two NCs compared to the base PP. These results indicate that plenty of mobile carriers and/or dipoles, probably resulted from the manufacturing process, remain in the two NCs. Notwithstanding the above-mentioned 'inferior' insulating properties, the two NCs have an improved PD resistance compared with the base PP. Namely, the erosion depth on the surface induced by PDs is the smallest in the NC with the largest filler content, while it is the largest in the base PP. Such differences in the effects of nanofillers on different insulating properties are attributable to the fact that nanofillers can improve the PD resistance simply by their presence, while the chemicals needed for uniform dispersion of nanofillers may sometimes increase the permittivity and abundance of charge carriers.