Abstract: A rigorous mathematical model of multistage flash system (MSF) is presented based on a detailed physicochemical representation of the process, including all the fundamental parameters such as temperature and concentration of brine water. In particular, three integer variables, which are the number of recovery stage, the number of rejection stage, and the ratio of recirculation brine water flow rate to distillation flow rate, are introduced to the model. The objective is to minimize the total annual cost, which is mainly composed of the operating costs and investment cost. Here the modified genetic algorithm (MGA) characterized as mixing coding way, is adopted for the system optimization. A case study and a discussion of the results are presented.