Reverse osmosis is a commonly used process in water desalination. Due to the scarcity of freshwater resources and wastewater problems, many theories and experimental studies have been implemented to optimize this process. In the present study, the performance of reverse osmosis membrane module of salt-water separation was simulated based on computational fluid dynamics technique and solutiondiffusion theory. Eight geometries of membrane modules, four flat sheets, and four tubular membranes were investigated. It was found that if the membrane surface area and inlet flow rate were kept constant for the eight modules, the pressure drop and permeated flow rate would be approximately similar for some geometries (e.g., the performance of primary flat sheet channel is the same as 3 tubular membranes with R=1/3 Rref). The results also showed that because of the phenomenon of concentration polarization, if it is possible to use more membranes with a smaller length, it can reduce the pressure drop and increase the permeation flux of water. Furthermore, the results showed that between the tubular and the plate membranes in similar conditions, the tubular one is more suitable for the water permeation due to its ease of construction and its ability to withstand ECP.