Solar desalination with a humidification-dehumidification process has proven to be an efficient method of utilizing solar energy for obtaining fresh water from saline water. The reason behind the success of this method is the use of latent heat of condensed water vapor. With proper use, this process efficiency could be high. A simulation program was written to describe the performance of such units. It was found to predict the performance of a desalination unit constructed in Jordan in 1993. The simulation program was then used to optimize the unit performance by studying the effect of different parameters such as the condenser and humidifier area, as well as the effect of some of the operating conditions such as the feed water flow rate to the unit.