This paper presents an optimization study of a once through multi-stage flash (MSF) desalination process with simple mixer. The optimization problem was to maximize the gained output ratio (GOR) at different plant capacities by varying the top brine temperature. A simple model developed by Soliman was used and slightly modified to take into account the variation of average boiling point elevation losses due to the increase in feed stream salinity resulted from recycling of brine in winter season. The optimization problem was solved using Microsoft Excel software utilizing the SOLVER tool. Optimal operating conditions were obtained at different plant capacities between 1000 and 1300 m(3)/day. (C) 2010 Elsevier B.V. All rights reserved.