The potential of using nanofiltration (NF) membranes in purification of the effluent from paper regeneration wastewater treatment plant was investigated. The change of filtration resistance for various NF membranes and the mechanisms responsible for the resistance were identified. The mahyority of the pollutants remaining in the wastewater were mostly nonbiodegradable and ware less than MWCO 3000 in size, indicating micro- or ultrafiltration may not be applicable. Among the five NF membranes used in this study, NTR-759HR showed the highest rejection efficiency; 98.5% and 99.5% for TOC and color, respectively. The TOC concentration of the permeate could be maintained at less than 0.7 mg/L regardless of the concentration of the feed solution. However, the permeate flux steadily decreased with time up to 50% of the initial value at the recovery rate of 89%, To investigate the possible mechanisms responsible for the reduction of permeate flux, the observed total filtration resistance was divided into three categories; (1) the intrinsic membrane resistance for pure water (R-m), (2) resistance due to concentration polarization (R-cp), and (3) resistance due to membrane fouling (R-f). The increase of filtration resistance for NTR 7410 could be explained by membrane fouling while those for NTR-759HR and NTR-7250 are attributed to the osmotic pressure build-up due to concentration polarization. The different mechanisms for the flux reduction were studied.