The occurrence of the limiting flux in ultrafiltration is elucidated with the formation of a cake (gel) layer on the membrane surface. Before cake formation, the pressure drop on the concentration polarization layer, as well as the permeate flux, increases with the applied pressure. The pressure drop on the concentration polarization layer, however, will no longer change with the applied pressure after the formation of the cake layer. The limiting flux will be obtained if the hydrodynamic conditions in the filtration channel are not affected by the cake layer. A mechanistic model for predicting the limiting flux in ultrafiltration is developed. A procedure to correlate the model with experimental ultrafiltration data is also presented.