The dielectric spectroscopy of three industrial nanofiltration (NF) membranes NF90, NF270 and NF-, before and after fouled by CaCO3, CaSO4, BaSO4, SrSO4, were measured. The fouling process and different Fouling mode caused by the different membrane pore radius were monitored by dielectric spectroscopy. The NF270 and NF- were mainly fouled in the inner of the membrane pore and NF90 was fouled both in the inner of the membrane pore and on the membrane surface where a scaling layer formed. By analyzing the dependence of dielectric parameters on the fouling time, the relaxation mechanisms of different membrane systems were explained: for the membrane NF270 and NF-, the relaxation observed at the initial stage of fouling process was the result of cooperation of interfacial polarization (between the fouled membrane and deionizecl water) and the concentration polarization (near the membrane surface) which is from the dissociation of the scale formed by insoluble salts (denoted as CP hereafter), and for that of membrane NF90, in addition to the above two mechanisms, the interfacial polarization between fouling layer and deionized water also contributes. The relaxation after 4 h was mainly controlled by the interfacial polarization between the fouled membrane and solution interface. Further, taking the membrane fouled by CaSO4 as an example, the dielectric analysis was carried out by introducing a constant-phase-element (CPE) to circuit to describe the fouled membrane and the scaling layer. Our Finding indicates that the size of membrane pore would influence the fouling process. The feasibility of dielectric spectroscopy as a noninvasive method to monitor the Fouling process of internal and surface of membrane was also discussed. (C) 2015 Elsevier B.V. All rights reserved.