A detailed study was performed to investigate the effect of annealing on the physical stability and conductivity of Nafion NRE 211 membrane. The result found annealing at a high temperature helps increase the crystallinity and improve the mechanical property of both H-form and Na-form membranes. The humidity-induced stress values of the membranes decrease accordingly for the H-form membrane annealed below 160 degrees C and the Na-form membrane annealed below 270 degrees C, which indicates that the physical stability of Nafion membrane can be improved to a durable state. However, annealing the H-form membrane always lowers the conductivity, while annealing the Na-form membrane enhances the conductivity because of the reorganization of the cluster structure in the membrane. The micro-structural change of the electrolyte membrane also investigated in detail. The results demonstrated the activity enhancement of the sulfonic groups and the pending side chains is responsible for the enhanced properties of the annealed membrane. (C) 2010 Elsevier B.V. All rights reserved.