The effects of electron-beam irradiation on the physicochemical and electrochemical properties of polyethylene (PE) separators are investigated. The high-energy electron-beam irradiation creates carbonyl bands on the surface of bare PE separators, however, it does not affect morphology and pore structure of the separators. In addition, cells employing the electron-beam-irradiated PE separators clearly exhibit better ionic conductivity and rate capability without any degradation in cycling performance compared to cells employing the bare PE separator. This improvement is explained by a formation of new functional group on PE surface—the electron-beam irradiation creates carbonyl group on the surface of the PE separator and it readily facilitates the migration of Li+ and improves solvent affinity of the PE separators. Furthermore, the thermal stability of PE separators is effectively enhanced by irradiating them with electron beams. The thermal shrinkage of the electron-beam-irradiated PE separators is observed to be much lower than that of bare PE separators, resulting in an increased gap between the shut-down and melting integrity temperatures. From these results, it is believed that the electron-beam irradiation can be considered as an effective approach to enhance electrochemical and thermal properties of PE separator.