Egypt faces the prospect of population growth pressing on available agricultural land resources. A very large areas of coastal desert could be helpful in population density redistribution and of great economical interest, with the installation of a suitable water supply. Of the many desalination processes, the Multi-Stage Flash (MSF) desalination process is still the most developed and reliable technology for large scale conversion of sea water to fresh water. In this paper, a computer code was developed for optimizing a large scale MSF plant. This code can calculate the optimum design for the plant, and the minimum cost of product fresh water, as a function of both design and operating variables. Sample results obtained using this code for the Egyptian conditions are presented. The effect of, energy and material costs, the chemical treatment types, and the feed water salinity on the optimum performance ratio and the minimum fresh water cost are drawn.