Perfluorinated ion exchange membranes were studied and the membrane technology for PEMFC has been developed. Thermal stability, mechanical strength, water content, AC specific resistance and gas permeability were measured. The influence of membrane thickness on gas permeability and the influence of incorporation of cations on water content and AC specific resistance of Flemion® and Nafion® 117 were estimated. Gas permeation rates of the membranes decreased in inverse proportion to the increase of the membrane thickness and gas permeability coefficients were nearly constant and independent of the thickness. Hydrogen permeation rates of Flemion®S at 70°C were converted to 2.1 mA/cm2 as cunent density. Flemion®R-electrode assembly showed to maintain stable perfonnance for over 3,500hr. Furthermore, it was found that usage of thinner membranes or one with higher ion-exchange capacity gave not only lower intemal cell voltage but also higher iR-free cell voltage. PTFE-yam embedded type membrane (Flemion®Mc and Sc) and PTFE-flbril dispersed type (Flemion®R12) was examined to afford improvement in mechanical strength at moist and high temperature atmosphere. Flemion®Sc (80!am) was examined to give high cell performance of 0.67V at 0.5A/cm2, 80°C, I ata. Flemior®Mc-electrode assembly was examined to keep stable performance during the life test of over 1,500hr.