Grafting of styrene onto nafion membranes was carried out by the impregnation and radical polymerization in the supercritical carbon dioxide (scCO₂) as a solvent and swelling agent. In impregnation process, certain amounts of CO₂ was charged into the apparatus until desired pressure was reached. Styrene monomer, initiator 2,2'-azoisobutyronitrile (AIBN) and crosslinker divinyl benzene (DVB) were impregnated into nafion membranes at 38 ℃ for 4 h. After releasing CO₂, the polymerization step then was started by raising the temperature to 80 ℃ and carried out at a pressure of 10 MPa for 4 h. The grafted membranes (N-g-ps) have been sulfonated to various degrees in concentrated sulfuric acid (98% H₂SO₄) at room temperature or 95 ℃. The sulfonated membranes (N-g-pssa) were characterized by measuring their ion exchange capacities (IEC), wateruptake, ion conductivity and methanol permeation. The structure and morphology of these membranes were observed with fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The permeability of methanol in nafion 115 was higher, with a value of 3.29×10^-6 cm²/s than that of a N-g-pssa membrane. The permeabilities of the N-g-pssa membranes decreased with increasing concentration of DVB, i.e. from 2.15×10 ^-6 to 1.74×10 ^-6 cm²/s. The N-g-pssa membranes (without DVB) has the best cell performance at 0.35 V because this membrane probably has the highest ion conductivity (0.0461 S/cm) and lower methanol permeability than that of Nafion 115.