In the present study, a natural circulation two phase closed thermosyphon flat plate solar water heater has been investigated theoretically under the actual field conditions of Cairo, Egypt. Also, the heater design parameters are optimized by means of the author's simulation program that was verified experimentally in a previous paper. These parameters include the ratio of storage tank volume to collector area, storage tank dimensions ratios and height between the heater storage tank and collector. The computational results' indicate that the storage tank volume to collector area ratio and the storage tank dimensions ratios have significant effects on the heater performance, while the height between the heater tank and collector has little effect.