A novel sulfonic acid (SO3H) group containing membrane (SH membrane) was prepared by grafting glycidyl methacrylate onto an ethylene-tetrafluoroethylene copolymer film with subsequent sulfonation. The graft chains introduced on the SH membrane form a flexible brush-type structure capable of binding water molecules (30% more than Nafion 117 membrane) and transporting protons. Proton conductivity of the SH membrane was evaluated as a function of SO3H group density. The SH membrane with the SO3H group conversion of 97% and the thickness of 70 mu m had specific resistivity of 24 Omega cm at 303 K, which was comparable to that of Nafion 117 membrane. A uniform distribution of the SO3H group across the SH membrane was verified using an electron probe x-ray microanalyzer.