Several self-cross-linked composite membranes (SCSP/SF) are constructed successfully through the combination of sulfonated poly(ether ether ketone) (SPEEK) containing propenyl groups with sulfonic group-functionalized graphene oxide (GO) and photochemical cross-linking of SPEEK. Tensile tests show that a significant mechanical reinforcement is obtained while the elongation at break of membrane increases with the functionalized GO loading, meaning the simultaneous improvement of strength and flexibility. The modified membrane with excellent mechanical properties may be more stable in the long-term operation of direct methanol fuel cells (DMFCs). Furthermore, the SCSP/SF membranes display enhanced thermal, dimensional, and hydrolytic stabilities than the pristine one. The optimum properties are presented by the modified membrane with a loading of 3% functionalized GO which gives a lowest methanol permeability (6.56 x 10(-8) cm(2) s(-1)) far below that of the pristine membrane and supreme conductivity (2.69 x 10(-2) S cm(-1)) above that of the pristine one at room temperature. This means that in the case of 3% functionalized GO content, the membrane possesses the best selectivity (4.10 x 10(5) S cm(-3)) which is about 6.8-fold and 19.3-fold more than that of the pure and Nafion 117 membranes. According to the above results, it could be concluded that the synergistic effect of self-cross-linking and functionalized GO makes the modified membranes potential in DMFC application. Based on our knowledge, no SPEEK-modified membranes have been designed using this method.