Surface modification of thin-film nanofiltration membranes was carried out to produce high water permeable NF membranes by Air-Ar plasma treatment. The effect of composition of used gases on membrane properties was investigated. Results showed that the plasma treatment decreased the water contact angle obviously from 80.4° to 6.5°, which in turn would increase the membrane surface hydrophilicity. The results of FTIR spectra decisively confirmed the formation of hydrophilic nitrogen and oxygen compounds on the membrane surface. The SEM images of membrane surface also showed significant changes after plasma treatment. AFM analysis indicated smoother surface for the modified membranes compared to pristine membrane; the roughness declined from 55.85 nm for virgin membrane to 28.33 nm for modified membranes. The salt rejection was 90% for pristine membrane and 76.35% to 92.45% for the plasma treated membranes. The water flux for modified membrane treated by 50% Air-50% Ar plasma increased ~1,446.1% compared to the virgin membrane, whereas the selectivity declined only ~15.1%.