Perfluorosulfonic acid polymer membranes, such as Nafion®, Aciplex®, and Flemion®, are attracting great attention as electrolyte membranes for fuel cells because they have high proton conductivity. In this paper, the effect of γ rays on the electrical properties of perfluorosulfonic acid membranes by applying a variable voltage and measuring the current and dimension of the sample have been investigated. In order to understand the irradiation effects on conductivity changes, we have used optical absorption measurement in the wavelength of 190 ∼ 500 nm. The results show that the electrical conductivity has been increased by increasing the absorption dose, and intensity of the absorption bands have increased by increasing the absorption dose. The electrical conductivities of the irradiated membranes at room temperature are enhanced about one order of magnitude higher than the conductivities of an unirradiated membrane. Moreover, an optical absorption measurement shows that the structures of the membranes are modified by γ radiation. Intensity absorption bands associated to fluorocarbon and peroxy radicals and C=O groups increased with increasing the dose. The change in electrical conductivity seems to be related to the high production of peroxy radicals in the irradiated membrane.