For the reason that a single stage RO-separation gives small concentration differences between given solutes, the use of multistage RO-separation is unavoidable with a two-or three-stage RO-processing with various recirculations, the system becomes rather complicated, so the search of the optimum (both in technical and economical viewpoints) is a necessary but difficult task. A relatively simple mathematical modelling method of the given separation process is discussed in order to find the optimum separation between sodium nitrate and boric acid in case of given membrane modules and input concentrations of components.