The impact of the geometry of spacer filaments on concentration polarization and permeate flux in spiral wound reverse osmosis modules was studied using the 2-D streamline upwind Petrov/Galerkin finite element model. In this study, both salt concentrations and flow velocities on membrane surface are determined as results of numerical solution of the coupled governing equations for momentum and mass transfers, which is more appropriate for most applications of the spiral wound membrane modules. The patterns of concentration polarization along the spacer-filled membrane channel were first investigated respectively for the membranes attached to and opposite to the transverse filaments. The patterns were then analyzed with the hydraulic characteristics (e.g., reattachment point and recirculation) of flow in the membrane channel for the implications to the system performance. Finally, the effect of filament shape and size of the spacer on concentration polarization and system performance was simulated and discussed. The results suggest concentration polarization in spiral wound RO modules could be significantly alleviated by spacer optimization.