In this study, a 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 collector in Kerman, Iran based on metrological data for a horizontal surface. The monthly, seasonal, and yearly optimum slope angles were found. The gained heats for a solar collector based on the monthly, seasonal, and yearly optimum slope angles were found. The results indicated that changing the monthly, seasonal, and yearly slope angles would achieve a yearly gain of 23.9, 22.6, and 14.5% in solar radiation intensity, respectively.