We have carried out dissipative particle dynamics (DPD) simulations in an attempt to better understand how molecular weight (MW) affects the hydrated morphology of the short-side-chain (SSC) perfluorosulfonic acid (PFSA) fuel cell membrane. Previously. we demonstrated that such coarse-grained simulations are capable of revealing differences in the morphology of PFSA membranes when either the length of the side chain or equivalent weight (EW) of the ionomer is changed [Wu et al. Energy Environ. Sci. 2008, 1, 284-293]. In the present investigation, the SSC ionomer was modeled using macromolecules of the ionomer with EWs of 753, 798 and 849, each at three distinct MWs. The morphological structures were then investigated as a function of EW, MW and degree of hydration (with water contents corresponding, to lambda = 5, 7, 9 11, and 16 H2Os/SO3H). Water contour plots reveal that the isolated water clusters present at lower water contents increase in size with increasing levels of hydration, and eventually form continuous water domains. The increase of MW induces aggregation of the fluorocarbon backbone in order to minimize chain bending forces while maintaining a phase-separated structure, and results in larger, more elongated water domains, especially at high EWs. Furthermore, the Bragg spacing corresponding to periodicity of water domains, computed from radial distribution functions (RDFs), shows that the spacing between water domains increases with increasing hydration levels. This occurs especially for higher MW polymers in high hydration (16 H2Os/SO3H), whereas there is little difference at lower hydration levels between polymers with different MW.