Correlation equations for predicting the permeate flux of membrane ultrafiltration in hollow-fiber modules were derived from momentum and mass balances with the application of the resistance-in-series model, coupled with the considerations of declinations of flow rate and transmembrane pressure along the fiber tubes. Experimental work for the ultrafiltration of Dextran T500 aqueous solution was carried out in an Amicon model H1P30-20 hollow-fiber module made of polysulfone. Correlation predictions are confirmed with the experimental results, especially for the practically operating range of transmembrane pressure. The effects of solution concentration, fluid velocity and transmembrane pressure, as well as tube diameter and length, on the declination of permeate flux along the fiber tubes were also discussed.