Styrenesulfonate grafted multi-walled carbon nanotubes (ss-MWCNTs) were prepared by a simple chemical reaction with soduim 4-styrenesulfonate to reinforce Nafion® membranes for use in direct methanol fuel cells (DMFCs). Although Nafion® membranes have excellent proton conductivity for fuel cell applications, methanol crossover through the Nafion® membrane remains a serious problem for DMFC applications. The prepared ss-MWCNTs had approximately 3.30 wt.% of sulfure and showed styrenesulfonate groups on the ss-MWCNTs. Then, the Nafion® membranes were reinforced with ss-MWCNTs to reduce methanol crossover. The styrenesulfonate groups on the ss-MWCNTs contained sulfonate end groups that enhanced miscibility of MWCNTs in the Nafion® membrance because of affinity of the same sulfonate groups in the ss-MWCNTs and the Nafion® membrane. Further, the phenyl structure of the styrenesulfonate groups on the ss-MWCNTs enhanced thermal stability at high temperature. The Nafion® membranes were reinforced with ss-MWCNTs (1 wt.%) using a solution casting with a certain amount of water and sodium 4-styrenesulfonate. Well-dispersed 1 wt.% ss-MWCNT reinforced Nafion® membranes were prepared, and the water and methanol uptake were investigated for DMFC applications. The methanol uptake value (36.84) of the 1 wt.% ss-MWCNT reinforced Nafion® membranes was reduced compared to that of the cast Nafion® membrane (38.85).