We know a phenomenon that partial discharge is generated before dielectric breakdown. We have been working on research for electromagnetic radiation due to partial discharge. We found that the observed frequency of the electromagnetic wave is characterized according to the partial discharge source. So we tried to analyze by a wavelet transformation in order to explain the mechanisms that produce a characteristic frequency. As a result，we found that frequency characteristics of the electromagnetic radiated from insulation appear at a peak of the electromagnetic signal. And we also confirmed that the frequency depends on electromagnetic and current pulse width at the peak. We carried out the dielectric breakdown test by using several specimens of epoxy resin. As a result，the characteristic frequency component of the radiated electromagnetic wave could not be seen at the start of charging，but we found the characteristic frequency component was observed with tree growth.