A new negatively charged nanofiltration (NF) membrane has been prepared by grafting of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the polysulfone (PSf) hollow fiber ultrafiltration membrane using two-step plasma method. First, PSf substrate was pretreated by plasma. Then, the pretreated membrane was soaked in AMPS solution and the loaded monomer was grafted on the membrane by plasma-induced. ATR-FTIR. FESEM. water contact angle (WCA) and breaking strength were employed to characterize the structural and morphological changes of membranes. The effects of pretreatment time, AMPS concentration and grafting reaction time on the membrane performances were studied. In addition, the rejections to single salt solutions and mixed salt solutions were examined. The results indicated that the pretreatment of PSf substrate played an important role in the preparation of this NF membrane. The NF membrane, pretreated for 30s and grafted for 80s with 5% AMPS solution, showed a rejection to 1 g/L Na2SO4 of 80.8 +/- 1.2% with a permeate flux of 6.4 +/- 0.2 L m(-2) h(-1) at 0.2 MPa and at 23 +/- 1 degrees C. The rejection sequence of single salts was Na2SO4 > MgSO4 > NaCl > MgCl2. And the negatively charged NF membrane showed a better selectivity to SO42-, than Cl- in a mixed salt solution. (C) 2011 Elsevier B.V. All rights reserved.