The crossflow filtration properties of monodisperse nanoparticle suspensions were investigated systematically by using flat ultrafiltration (UF) membranes for average nanoparticle diameters of 10, 45, and 80 nm. The relationships between (1) permeate flux and filtration time, (2) steady-state flux and applied pressure, and (3) limiting flux and feed concentration were studied under various experimental conditions. These relationships were found to be similar to those observed for the UF of aqueous macromolecular solutions, although the particle size of our target material was much larger than that of such macromolecules. In addition, the experimental data were analyzed by using the concentration polarization model, which is based on the mass balance beneath the membrane. As in the case of aqueous macromolecular solutions, the gel-layer concentration or limiting flux at low feed concentrations could not be explained well by the abovementioned model. Nevertheless, we demonstrated that UF membrane separation has the potential to be an effective processing technique for nanoparticle suspensions.