Abstract: The saturated permeability tensor of composite preforms was predicted by the combination of homogenization theory and computational fluid dynamics technique. Generalized Darcys law was first derived from the study of fluid flow through a solid-fluid mixture, and the saturated permeability tensor of a preform could be predicted by studying its unit cell under periodic boundary conditions. The efficiency of this method was validated by conventional analytical methods for unidirectional fabrics. For preforms with complex microstructure, the permeability prediction involved two steps. First, the effective permeability values of the open channel and tows of the unit cell were determined respectively, and the overall permeability was determined accordingly. For a plane-weave fabric, the results obtained here are in good agreement with other methods and experimental results. The effects of unit cell microstructure were also investigated. The analyses show that the permeability values may differ from each other significantly for preforms with equal porosities, similar microstructures but different inter-tow channel configurations.