The effects of operating conditions on the permeate flux for ultrafiltration of aqueous solution of PVP-360 in hollow-fiber membrane modules, have been investigated based on the resistance-in-series model coupled with the gel-polarization model under the consideration of the declination of transmembrane pressure along the axial direction in the hollow fibers. It is seen that correlation prediction may be improved if the more precise, but rather complicated, integral equation derived can be solved and if more experimental data are available.