In this paper, the distribution of the electric field of medium voltage (MV) underground cable insulation made of polyethylene/copper nanoparticles (XLPE/Cu) is investigated. Three methods were established for copper nanoparticles (CuNPs) synthesis such as the alkaline reduction process, electrolysis process and metal displacement reaction. Ultraviolet–visible spectra, X-ray diffraction, and transmission electron microscopy (TEM) were used to classify the obtained CuNPs. The XLPE/Cu films with different CuNP weight content (0%, 1%, 3% and 5%) were subjected to a 3 MeV electron beam (EB) with different doses (0, 15, 20 and 25 kGy). The optimum AC/DC conductivity (AC 2 × 10−3 S m−1, DC 4.63 × 10−2 S m−1) in minimum relative permittivity (2.05) achieved for (XLPE/5-Cu) 25 kGy that is has a CuNP content of 5 wt-% and irradiated at 25 kGy. The electric field distribution in the middle of the (XLPE/5-Cu) 25 kGy sample is maximum uniform than at the top or bottom.