A generalized analytical model was developed to determine the optimum tilt angle of flat-plate solar collectors for any location worldwide. The developed analytical model was found to be directly related to the latitude phi of the location under consideration, the declination angle delta of the earth axis, and the day hour angle at sunset at any day throughout the year. The optimum tilt angle beta(opt) was mathematically expressed by beta(opt) = phi - tan(-1) [{tan delta(pi omega(s)/180)}/sin omega(s)]. This model was applied to Riyadh, Kingdom of Saudi Arabia. The results were compared with those calculated by NASA, and showed very good agreement.