Abstract The proton dynamics in Nafion ®117 is investigated by comparison of the diffusion coefficient D NMR estimated from PFG-NMR with that of D σ estimated from electrical conductivity. At high water content region, D σ is about two times higher than D NMR as a result of Grotthuss mechanism. At low water content region, D σ and D NMR are in good agreement with each other. Both of the diffusion coefficients decrease steeply at low water content region. It can be explained as a result of the percolation transition due to the isolation of ion clusters, which is suggested by the recent small angle X-ray scattering data.