A random poly(ethylene-co-acrylic acid) (PE-A) with an acrylic acid (AA) content of 5.0–20mol% was functionalized by esterifying acrylic acid group with poly(ethylene glycol) monomethyl ether. Polyethylene oxide functional groups such as a pendant were introduced into the polyethylene backbone chain. The resulting polymer (PEGM-g-EAA) can be easily formed to a thin sheet and possesses the adhesion property such as gluing. Its thin film could absorb and hold a large quantity of the electrolyte solutions employed for the lithium batteries. The ionic conductivity of the PEGM-g-EAA gel electrolyte obtained with the starting PE-A with acrylic acid content of 9.0mol% was a value of around 1.5×10−3 Scm−1 at 20°C. The ionic conductivity results obtained for the network type gel, which was entangled with the present PE-A-based polymer, were 1.1×10−3 Scm−1 and 5.5×10−3 Scm−1 at 0°C and 80°C, respectively. The characteristics of good thermostability, transparency and good adhesion to the electrodes have also been demonstrated. As an example, the test cell consisted of the proposed polyolefin gel electrolyte, a LiCoO2 cathode and a lithium anode showed excellent charge/discharge characteristics.