Polypropylene has been used in radio-frequency capacitors and power cables as composite insulating material. While being mixed with natural clay nano-particles, it can be used as a nanocomposite insulating material in practical engineering. The objective of this study is to evaluate the electrical properties of polypropylene filled with natural clay as nano-material. A comprehensive test platform for testing electrical breakdown characteristics of material samples is setup. Polypropylene samples having 0%, 2% and 6% of natural clay by weight are exposed to voltages at two different rates-of-rise while measuring the RMS current. Breakdown voltages of each samples are also determined at two different ramp-rates. The important findings of this study are:1) The breakdown strength of the natural nanoclay filled polypropylene is higher than the unfilled polypropylene and the optimum concentration of nanoclay appeared to be 2% by weight; 2) Within the scope of the experimental samples in this article, the addition of nanoclay particles at 2% will significantly improve the electrical properties of the material. The results of this paper can provide reference for the application of polypropylene composite insulation materials.