The nano-scale morphology of a proton exchange membrane (PEM) strongly influences its proton conductivity and mechanical performance. In this paper, a multi-scale modeling approach has been developed to obtain the morphologies of hydrated perfluorosulfonic acid (PFSA) membranes and then to predict their mechanical properties based on the simulated morphology. Two representative ionic domain morphologies were compared, spherical and cylindrical, to represent cast and extruded membranes. The calculated overall elastic Young's moduli are very close for both morphologies and agree well with experiments. The nano-scale phase segregation in hydrated PFSA induces a non-uniform distribution of local stress. The peak stress is localized at the smeared water/PFSA interface. The cylindrical morphology develops much lower peak stress than the isotropic spherical morphology under the same level of strain. These results explain why the extruded membranes show more than 10 times longer life in durability tests than recast membranes, despite having similar bulk moduli. (C) 2010 Elsevier Ltd. All rights reserved.