The effects of electron beam (EB) irradiation on dielectric breakdown voltage (BDV) and insulation resistance (rI) changes of poly (ethylene-co-tetrafluoroethylene) (ETFE) insulated electric wire were investigated. Electric wire was prepared by extruding pristine ETFE. The samples were irradiated in air at room temperature by a universal EB accelerator by doses ranging from 1 to 20 Mrad. The thermal behaviour of extruded samples has been also investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). When the ETFE insulated wire was exposed to EB irradiation, BDV and rI were decreased gradually with irradiation dose. As the irradiation dose increased, the char formation and size of breakdown point were decreased significantly (above 10 Mrad). In TGA study, the thermal stability of EB-irradiated ETFE samples decreased significantly with radiation dose. For ETFE-20 the TGA traces showed a shift of the weight loss towards lower temperature with stabilization of 50°C lower than pristine ETFE. DSC Melting temperature (Tm) and crystallization temperature (Tc) of EB-irradiated ETFE tended to decrease as the irradiation dose increased. The crystallinity (c) was slightly increased up to 5 Mrad irradiation dose and decreased significantly with irradiation dose above 5 Mrad. An increase in c values occurred due to chain scission in the amorphous region, which led to crystallite perfection. The chain scission process created shorter polymer chain links which became easier to fit into a localized crystalline domain. At higher EB irradiation dose, the crystalline structure tended to be destroyed which led to decreases in Tm and Tc. After EB irradiation, the tensile properties and scrape abrasion resistance of ETFE were also decreased with respect to pristine ETFE.