Nafion membranes have been modified by PGMA using persulphate initiation system, subsequent by sulfonation of PGMA graft branches, to convert the epoxy groups into sulfonic groups, which provide the modified membranes with more acidic sites to maintain its ionic conductivity. The grafting process and the sulfonation process have been confirmed by FT-IR, TGA and FT-IR, EDAX analysis and ion exchange capacity measurements, frequently. Moreover, the physicochemical properties of the modified membranes, such as water and methanol uptakes, and ion exchange capacity, have been studied. The results showed that the modified membranes have high water uptake. The ion exchange capacity measurements proved the conversion of the epoxy groups into sulfonic groups in the sulfonated grafted membranes since the sulfonated grafted membranes showed ion exchange capacity higher than that of grafted membranes. Also, the results showed that the modification process has no impact effect on the stability of the membranes dimensions in water and methanol. TGA analysis showed that the modified membranes exhibited high thermal stability than that of unmodified Nafion membranes. SEM analysis showed the homogeneity of the PGMA in the matrix of Nafion membrane. The methanol permeability of the modified membranes decreased with increasing the polymer content in the modified membranes until reached to minimum value 1.49 x 10(-6) cm(2)/s of grafting percentage 18.76%, which helped to reduce the methanol permeability by 45.36% in addition to higher thermal stability and higher water uptake. The modified membranes showed high-performance factor (584 x 10(-9)) in comparison to unmodified Nafion membranes (309 x 10(-9)).