A study on the breakdown of polypropylene filled with synthetic nano-clay is presented. The synthetic clay contents were 0 %, 2 %, 4% and 6 % by weight in host polypropylene. After preparing each specimen a power-frequency, sinusoidal voltage was applied to the specimen with a well-defined ramp-rate up to a level at which a breakdown occurred. The breakdown voltage of the specimen was considered as the measured rms voltage at the breakdown. The mean breakdown strength seems to improve about 13.5% for the nanocomposite with 2% synthetic clay as compared to the plain polypropylene. Although improvements were observed on the breakdown strength of nanocomposites with higher contents of synthetic clay the improvement was not as dramatic as was the case between the 0% and 2% contents in polypropylene. Thus, it can be concluded that the nanocomposite with 2% synthetic clay content is the optimum. Moreover, an overall improvement was determined on the breakdown strength of the nanocomposites with the increase of the voltage ramp-rate.