Composite membranes were formed by grafting styrene using supercritical carbon dioxide (scCO2) impregnation and polymerization procedures. A polypropylene membrane, styrene monomer, divinylbenzene (DVB), and 2,2'-Azoisobutyronitrile (AIBN) were placed in a reactor, and CO2 was injected into the reactor at 38 ℃ for the scCO2 impregnation process. After impregnation, the polymerization process was carried out at 78 ℃. The grafted membranes were sulfonated in concentrated sulfuric acid at 95 ℃. These polypropylene grafted polystyrene sulfonic acid (PP-gpssa) membranes were characterized by using various methods. The morphology and structure of the PP-g-pssa membranes were analyzed with scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and energy dispersive spectrometer (EDS). The ion conductivity and methanol permeability were also measured. The ion conductivity of the PP-g-pssa membranes was higher and the methanol permeability of the laminated membrane was lower than that of Nafion membranes. The performance of the PP-g-pssa/Nafion laminated membranes was evaluated in a DMFC unit cell at 90 ℃.