New multistage flashing desalination units (MSF) are being constructed with large capacity that may vary from 50,000 to 75,000 m(3)/d. This is almost 2-3 times the conventional unit capacity of 27,000-32,000 m(3)/d, which were common for the units installed in the 1980's. Most of the Gulf States and several countries across the world are acquiring the large units to take the merits of reduced product cost caused by the large unit capacity. Literature studies and field reports indicate that the unit product in these units is almost identical to seawater reverse osmosis and the multiple effect evaporation, with an average of $0.5/m(3). This paper focuses on modeling and simulation of the performance characteristics of these large units. Mathematical modeling of the MSF process is well established in the literature. However, several of the design parameters have to be adjusted in order to take into considerations the characteristics of the large unit capacity. The analysis focuses on evaluation of the weir loading, the dimensions of the condenser tube bundle, demister dimensions, stage dimensions, and temperature and flow rate profiles. The model predictions are validated against field data for a number of existing units.