Recently, Nanofiltration (NF) membranes have been employed in pre-treatment unit operations in both thermal and membrane seawater desalination processes. This has resulted in reduction of chemicals used in pretreatment processes as well as lowering the energy consumption and water production cost and, therefore, has led to a more environmentally friendly processes. In order to predict NF membrane performance, a systematic study on the filtration performance of selected commercial NF membranes against brackish water and seawater is required. In this study, three commercial nanofiltration membranes (NF90, NF270, N30F) have been used to treat highly concentrated (NaCl) salt solutions up to 25,000 ppm, a salinity level similar to that of seawater. The membranes were firstly characterized using the Atomic Force Microscopy (AFM) technique. Pore size and pore size distribution obtained from AFM measurements were used to analyse both experimental data of pure water permeation and data obtained from salt rejection. The main parameters studied in this paper are feed pressure and salt concentration. The experimental data of this work was correlated and analysed using the Spigler-Kedem model. In particular, the reflection coefficient (sigma) of all studied membranes and the solute permeability of the salt (P-s) have been determined for all membranes and at all salinity levels studied. For a salinity of 5000 ppm and a pressure of 9 bars, the experimental results showed that NF90 could achieve a salt rejection up to 95%, whereas its rejection dropped to 41% at a salinity of 25,000 ppm and the same pressure. Rejection levels achieved by NF270 have been in range of 11-29%, while N30F gave the lowest rejection in the range of 3-6%.