A modified membrane for vanadium redox flow battery (VRFB) was prepared by grafting sodium 4-styrenesulfonate (NaSS) to the surface of Nafion 212 via the oxygen plasma induced grafting technique to reduce the crossover of vanadium ions of the membrane. By grafting with poly(styrenesulfonic acid) (PSS), the crossover of vanadium ions through the Nafion membrane is only 32% of that of the pristine Nafion membrane. Thus, comparing with pristine Nafion membrane, the resulting Nafion-g-PSS membrane exhibits higher ion exchange capacity (IEC) and through-plane conductivity. In addition, this Nafion-g-PSS membrane improves the voltage efficiency (VE), coulombic efficiency (CE), and energy efficiency (EE) of VRFB. Owing to lower vanadium ions permeability, the VRFB with Nafion-g-PSS membrane exhibited much slower self-discharge than the VRFB with pristine Nafion 212, and the VE, CE, and EE after 200 cycles of charge-discharge test remain stable. In particular, the VRFB with Nafion-g-PSS membrane exhibits higher capacity retention than the VRFB with pristine Nafion 212 membrane.