In this study breakdown characteristics such as dielectric breakdown strength(EB) and breakdown phenomena, the form of electrical degradation known as electrical trees and life characteristics have been widely studied by applying AC high voltage to the treeing test needle imbedded DGEBA/MDA/SN system as well as changing temperature from0 to 200℃. It shows that EB decreased abruptly at HDT and the breakdown behavior above Tg supports that the amount of energy applied to DGEBA/MDA/SN system and the heat discharge from them are very important for electrical/thermal composite breakdown mechanism. These results are discussed in detail.