The performance of a solar collector is highly susceptible by its slope angle with the horizon. This is due to the fact that the solar radiation intensity over the solar collector surface changes with the slope angle. A well known mathematical model was used for estimating the total (global) solar radiation intensity on an inclined surface faced to the south, and to determine the optimum slope angle for the solar collectors in hot and dry parts of Iran based on experimental data for a horizontal surface. The monthly, seasonal, and yearly optimum slope angles were calculated. The slope angle was calculated for different intensities of total radiation on the collector surface and the corresponding values for maximum total radiation were specified as the optimum slope angles. The results showed that the yearly optimum slope angle for each city is close to the local latitude. The results also indicated that changing the monthly, seasonal, and yearly slope angles causes to achieve a significant yearly gain in solar radiation intensities when the gains are calculated with respect to a solar collector fixed at zero slope angle for the region under investigation.