The demand for lowering the overall costs is one of the main factors driving the expanding market for alternative materials in high-voltage insulating components. Polymer insulators provide several benefits as compared with conventional materials such as glass and ceramic insulators. In addition, the benefits of nanotechnology through different application, this paper study and evaluate the impact of zinc oxide nanoparticles on the thermal, mechanical and electrical behaviors of high-voltage electrical insulations composed of ethylene propylene diene monomer (EPDM) with and without a certain gamma dose. ZnO nanoparticles were introduced to the EPDM sample at different weight percents (0%, 0.1%, 0.3%, and 0.4%). Scanning electron microscopy (SEM) was used to analyze and characterize the microstructure of samples. Measurements of the mechanical and thermal properties of the formed composites were performed. After that, the prepared samples were tested for dielectric properties, which included the AC breakdown strength (BDS) and dielectric loss factor at room temperature. In addition, the same dielectric properties are measured for different aging period without and with radiation level of 20 kGy. All EPDM nanocomposites have an improvement in mechanical and thermal properties with an increment in the concentration of ZnO nanoparticles up to 0.1% and gamma doses up to 20 KGy. With increasing ZnO concentrations up to 0.3% and gamma doses up to 20 KGy, AC BDS increases.