Membrane bio-fouling is a major problem in membrane filtration operation. In order to reduce biofouling, we create a zwitterionic surface on the nanofiltration membrane. Nanofiltration membranes were fabricated by interfacial polymerization of trimesoyl chloride and diethylenetriamine. The surfaces were modified by N-alkylation of the secondary amines with iodopropionic acid. The resulting tertiary amines were further quarternized by iodomethane. The quarternized zwitterionic Q-IPA25 membrane, owning a near neutral zeta potential at pH 7.1, has excellent resistance to the adsorption of negatively charged bovine serum albumin and positively charged egg white lysozyme. The membrane, having a zeta potential higher than the unmodified one, was also capable of reducing the adhesion of negatively charged bacteria, Gram negative Escherichia coli and Gram positive Staphylococcus epidermidis. Fouling by humic acid was also considerably reduced. The membrane permeability and salt rejection properties were only slightly altered because of the change of surface zeta potential. It indicated that the interfacial zwitterionization was a suitable method to reduce bio-fouling on nanofiltration membranes. (C) 2011 Elsevier B.V. All rights reserved.