A computational model for RO system design and performance prediction was developed in this study. The model was developed to estimate the performance parameters of RO in a multiple membrane elements pressure vessel. For simplicity the results of this study were compared with Reverse Osmosis System Analysis (ROSA) software which was assumed to have acceptable marginal errors. Two different feed water (NaCl) concentrations 35,000 mg/L and 38,000 mg/L were investigated in this study. In this paper, the recovery rate, salt rejection, feed pressure and permeate concentration of each RO element in the pressure vessel was compared with ROSA as shown in figures 2 to 5. The results from this study showed a very good agreement with ROSA up to 95%. Most of the previous studies were focused on studying the performance of a single RO membrane. This study, probably, the first to present a systematic procedure for estimating the performance of multiple RO elements in a pressure vessel. (C) 2012 Elsevier B.V. All rights reserved.