Polyvinyl alcohol (PVA) was incorporated into composite nanofiltration membranes in situ by adding different amounts of PVA into piperazine (PIP) during its interfacial polymerization with trimesoyl chloride (TMC). Effect of PVA on membrane structures, nanofiltration performances and anti-fouling properties was studied by X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and cross-flow membrane module. It was found that the flux was increased without sacrificing rejection after adding PVA. XPS results show that the active layer contains PVA and polyamide components. AFM and SEM show that the roughness of these membranes decreases proportionally with increasing the mass fraction of PVA to PIP (the root mean square roughness value (RMS) decreases from 110.7 nm to 41.6 nm after adding 16% PVA). The hydrophilicity and flux of the composite NF membranes also increase with increasing the mass fraction of PVA to PIP. Moreover, the anti-fouling performance of composite polyamide NF membranes is improved with introducing the PVA into membranes. Therefore, PVA as an additive reagent has the potential to improve the nanofiltration performance of polyamide composite membranes. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.