In this paper, polysulfone (PSF)-Fe3O4 Composite ultrafiltration membrane was made by phase-inversion process and its structure of surface and cross-section was examined by scanning electron microscopy. Especially its ultrafiltration performance in magnetic field was studied by determining the variation of rejection to lysozyme. As a result, the addition of nano-sized Fe3O4 particles had an important influence on membrane performance in magnetic field. The rejection to lysozyme declined obviously in magnetic field, however, when the magnetic field was moved away, the rejection made a comeback quickly. While the rejection of the PSF membrane remained invariable in magnetic field. Furthermore, with the increase of the magnetic intensity, the rejection declined more obviously. Hence, the result indicated that it was possible to separate different substances with a composite membrane in turn by altering the magnetic intensity. (c) 2006 Elsevier B.V. All rights reserved.