The effects of space charge on dielectric breakdown phenomena in the epoxy resin system DGEBA/MDA modified with GN was observed. As the temperature increased the dielectric breakdown strength decreased rapidly at higher than HDT. At high temperature the electron was emitted and flew further than low temperature, easily. The emitted electron from the magnified electrode tip flew through the polymeric chain, trapped deep inside and formed space charge. The space charge distorted the electric field and concentrated it on the weakest point and decreased electrical properties. The eroded surface of tree channel by thermal electron emitted was detected.