For the improvement of organic electrolyte holding ability, the hydrophobic surface of a porous polyethylene (PE)-membrane separator was modified by grafting a hydrophilic monomer, glycidyl methacrylate (GMA), PE-g-GMA, by using electron beam technology, and applied to a thin film type Li-ion battery to elucidate the effect of a surface modification of a PE membrane separator on the cyclic life of Li-ion batteries. The Li-ion battery using the PE-g-GMA membrane separator showed a better cycle life than that of the unmodified PE membrane separator, indicating that the surface hydrophilicity of the PE membrane separator improved the electrolyte holding capability between the electrodes in the Li-ion cell and prevented the electrolyte leakage.