TECHNOLOGY FOCUS - ELECTRONICS - Preferred Method: An electrode assembly of a fuel cell, is formed using the polymer electrolyte film.    TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The polymer electrolyte film further has a multiple crosslinked structure. The high polymer film base material consists of an olefin-type polymer or fluorine-type polymer. The vinyl monomer solution consists of vinyl monomer having phenyl group and vinyl silane coupling agent which does not have phenyl group. Preferred Methods: The graft molecular chain is formed by combining vinyl silane coupling agent with vinyl monomer solution or combining polyfunctional vinyl monomer with vinyl monomer solution. The method further involves forming multiple crosslinked structure by performing an irradiation process with gamma -ray, an electron beam or X-ray, after carrying out the silane crosslinked-structure addition process. The graft polymerization process is carried out by irradiating with an ionizing radiation. EXAMPLE - An ethylene tetrafluoroethylene copolymer (ETFE) film was cut in dimension of 5 cmx 5 cmx 50 mu m and introduced into a glass-made separable container. The container was then deaerated and substituted with argon gas. The ETFE film was then irradiated with gamma -ray. Subsequently, a solution of p-styryl trimethoxysilane (8 g) and toluene (12 g), containing a vinyl silane coupling agent was added into the container and reacted at 60degrees C for 24 hours. The obtained graft-polymerized film was washed with toluene and dried. The dried graft-polymerized film was immersed in 0.2M chlorosulfonic acid diluted with 1,2-dichloroethane and reacted at 80degrees C for 10 hours. The obtained electrolyte film was immersed in 1M hydrochloric acid aqueous solution and condensed at 80degrees C for 24 hours, to obtain a polymer electrolyte film. The obtained polymer electrolyte film had graft ratio of 36%, ion-exchange capacity of 0.37 mmol/g, proton conductivity of 0.015 ohm-1.cm-1, moisture content of 11% and oxidation resistance of 350.