Recent studies have shown that NF membranes can effectively remove natural organic matter (NOM). However, NOM also serves as an organic foulant to membranes, resulting in flux decline. In this study, the flux decline of NF membranes was analyzed through the transport characterization of NOM in NF membrane pores. The transport of NOM through NF membrane pores was influenced by either convection or diffusion, depending on the applied hydrodynamic operating conditions, as represented by the ratio J(0)/k (J(0) represents the initial pure water flux, and k is the mass transfer coefficient) [1]. Different behaviors of flux decline, and flux recovery of tested membranes could be obtained: depending on which transport mechanism (either convection or diffusion) dominates during NOM filtrations. The removal (or rejection) behavior of NOMs, by NF membranes, was evaluated with respect to the ratio J(0)/k under a dominant transport condition, along with the performance of the flux decline effects.