A number of investigators have made efforts to develop various analytical models for hollow fiber type reverse osmosis (RO) module systems since the 1970s. However, a perfect analytical model, which c~m precisely explain the observed RO performances under a wide range of operating conditions has not been developed yet. The author previously proposed d precise analytical model called a friction-concentration-polarization medel (FCP model) [1], which used the Kimura-Sourirajan model for transport phenomena of solute and water transport through a membrane, taking a mass transfer coefficient as local variables and taking a fiber-bore side pressure drop into account. In the application of this model, fundamental transport pananeters of hollow fiber membranes were needed, and they were initially determined by a U-tube membrane test where the effect of concentration polarization could be neglected. Then a local mass transfer coefficient was estimated from experimental data using actual modules as a function of Reynolds and Schinidt numbers by a trial-and-error method fยข.: both brackish water and seawater desalination cases. Using all of the above results, behaviors of hollow fiber modules under various operating conditions were estimated and compared with the results obtained from commercial size module experiments. Compared to other previous models, the FCP model is verified to be the best one to predict actual module performances. This model will be further extended to analyze a change of transport paramcte~s during long-term runs.