Earlier work at Saline Water Conversion Corporation (SWCC) R&D Center showed that nanofiltration (NF) pretreatment of seawater feed to seawater desalination plants removed from it turbidity and microorganism, caused significant rejection of the scale forming hardness ions, reduced TDS in Gulf seawater and produced a new, partially desalinated seawater product, considerably different and superior to seawater in qualities and without the problems normally associated with seawater of high concentration of scale forming ions, high TDS, high turbidity and high bacteria count. Integration of the NF unit with one of the conventional desalination processes to form for example an NF-SWRO lead to a significant improvement in the seawater desalination processes, for example by doubling the SWRO product water output and recovery ratio and the production of high purity permeate (TDS < 200 ppm) from one single stage SWRO. However, all previous works were done on a pilot plant scale by integrating NF spiral wound membrane elements in series, size 4 " x40 ", with a SWRO unit employing 2.5 " x40 " membrane elements. In view of the positive and encouraging results obtained on a pilot plant scale, trial on an NF-SWRO demonstration unit was not only logical but essential to determine the operating conditions as well as to establish plant performance parameters for commercial size NF-SWRO plants. The demonstration unit built for this purpose consisted of 6 NF spiral wound membrane elements, size 8 " x40 " arranged in series followed by a SWRO unit comprising 3 HFF SWRO elements 8 " x40 " or 9 " x30 " also arranged in series. The paper describes the NF-SWRO demonstration unit along with the experimental approach used in this investigation dealing with the performance of several selected NF membranes integrated with SWRO membrane to form an NF-SWRO desalination system. The results obtained from the demonstration unit confirm the earlier results which were obtained from the pilot plant study. The results shall be utilized in the operation of the now under construction NF-SWRO plant at Umm Lujj, Saudi Arabia.