A correlation equation for predicting permeate flux of membrane ultrafiltration in hollow-fiber modules has been derived based upon the resistance-in-series model with the consideration of transmembrane pressure declining along the tube axis of the hollow fibers. Ultrafiltration of a PVP-360 aqueous solution has been carried out in an Amicon model H1P30-20 hollow-fiber cartridge made of polysulfone. Correlation predictions are confirmed with the experimental results. Ultrafiltration in four combined-module systems arranged with three identical modules has been further investigated theoretically. It was found that performance in the module system arranged in series is largely better than that arranged in parallel, except for the solutions of higher feed concentrations (1.0 wt% less than or equal to C less than or equal to 2.0 wt%) operated under low transmembrane pressure.