In order to control surface pore sizes of polyacrylonitrile (PAN) ultra filtration membranes and their distribution, the present work concerns the graft-modification of membranes by Ar low-temperature plasma. Some typical plasma irradiation conditions such as plasma power of 30-40 W, irradiation time of 60 s and pressure of 15-20 Pa were experimentally selected by our previous experiments. N-vinylpyrrolidone (NVP) monomers mixed in water were grafted onto the PAN membrane surface in this investigation. The effectiveness of grafting was systemically studied under different graft reaction conditions, such as concentration of monomer, reaction time and reaction temperature. The chemical changes at the modified surface were characterized by reflection FTIR, X-ray photoelectron spectroscopy (XPS). The permeation of the prepared nanofiltration membranes were studied using pure water. Water flux significantly declines with increase in both graft reaction time and temperature of grafting medium. In the low concentration range studied, NVP monomer concentration did not have strong effect on flux. At 2 MPa and 30 degrees C filtration conditions, the grafted membrane has a retention of 83.5% to salts for a mixed salt aqueous solution (0.03 MMgSO4 + 0.02 MNaCl).