The decline of permeate flux for ultrafiltration along membrane tubes was predicted from mass and momentum balances, based on the resistance-in-series model coupled with the considerations of the declinations of flow rate and transmembrane pressure along the membrane tubes. It is concluded that the declination of permeate flux occurs mainly due to the decline of transmembrane pressure and the increase of the thickness of concentration polarization layer along the membrane tube, and thus it is rather sensitive for lower transmembrane pressure and larger solution concentration.