This study presents the breakdown behavior of polypropylene filled with a various concentrations of nano-size natural clay. The concentrations of natural clay by weight were 0 %, 2 %and 6 % in polypropylene. For different concentrations of natural clay each specimen was subjected to a sinusoidal voltage waveform at power frequency with a certain ramp rate to a level where a breakdown occurred. The rms voltage at the breakdown was considered as the breakdown voltage of the sample. The mean dielectric strength improved about 3.5 % for the nanocomposite with 2 % of natural clay concentration in the host polypropylene. However, the improvement on the mean dielectric strength of the nanocomposite with 6 %concentration of natural clay appeared to be little more than 1 % when compared with 2 % concentration of natural clay in the host. This implies that there exits an optimum concentration of nanosize natural clay in polypropylene. Also observed was the increase of the dielectric strength of the materials with the increase of the ramp rate of the applied voltage.