The use of seawater desalination plants using RO technology has spread and the scale of the plants has increased. In such a situation, a larger-sized RO module has been strongly required. The conventional hollow fiber type RO element is a single open-ended (SOE) structure. That is, one side of the hollow fibers in the modules is opened and the other side is closed. In this SOE structure, the increase in the flow pressure loss of the permeated water which flows in the bore side of the hollow fibers prevents development of a large-sized (longer) RO element. In this work, a both open-ended (BOE) element was devised which can reduce the flow pressure loss of the permeated water. It has been confirmed by analysis and experiment that the permeate flow rate of BOE is greater by about 30% than that of SOE. Furthermore, the large-sized RO module with high volume efficiency was designed using the performance analysis method that was confirmed to be applicable to BOE. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2267-2277, 2008