In this study the effect of nanometer-size mica fillers in polyurethane is investigated by evaluating electrical breakdown voltages. The particle size of mica fillers used in this study is 1 nm and 10 nm. The TPU-mica composites were prepared with various concentrations of filler content. Each composite sample was subjected to a sinusoidal voltage waveform having a frequency of 60 Hz at a certain rate of rise up to a level where a breakdown occurred. The corresponding rms voltage at this breakdown level was recorded as the breakdown voltage of the sample. Except mica contents of 2% and 10% by weight, the breakdown voltage of the composite was found to be higher with 1 nm particle size than those with 10 nm particle size. The nanocomposite with 1 nm particle size and a concentration of 1% by weight indicated a breakdown voltage almost twice the breakdown voltage of pure TPU.