In this study, a mathematical model was used for estimating the total (global) solar radiation on an inclined surface faced to the south, and to determine the optimum slope angle for the solar collectors in Isfahan (central part of Iran) based on experimental data for a horizontal surface. The monthly, seasonal, and yearly optimum slope angles were found. Effect of earth reflection on the maximum total solar radiation was investigated. The results showed that neglecting the earth reflection decreases the optimum slope angle. The results also indicated that changing the monthly, seasonal, and yearly optimum slope angles causes to achieve a significant yearly gain in the solar radiation for the region under investigation.