Detection of electro-magnetic waves is one of a good diagnosis tool that can be used for the condition assessment of insulating systems. Detection and frequency analysis of electro-magnetic wave caused by partial discharge in epoxy resin have been investigated. Electro-magnetic waves radiated by partial discharge could be detected by using simple system with a conventional discone antenna and a spectrum analyzer. Detected electro-magnetic wave spectra have two specific frequency regions: one is at lower than around 200MHz and another is higher than around 200MHz. From observation of electrical tree development and measurements of discharge current and partial discharge level, the former is associated with a void discharge in epoxy resin and the latter is identified as tree development. By using these specific frequency regions, it is applicable to detection of partial discharge and insulating assessment for power apparatus and their accessories including epoxy resin.