In this article, the daily, monthly, seasonally, and yearly optimum slope angles of solar collectors are determined for areas of Iran, and new correlations are developed to calculate the monthly, seasonally, and yearly optimum slope angles for latitudes of 20 degrees to 40 degrees North. To achieve this purpose, the values of monthly optimum slope angles of different locations in the above range of latitudes are needed. Thus, the slope and surface azimuth angles of solar collectors for receiving maximum solar radiation are determined in some Iranian cities in different days, months, seasons, and the whole year, employing different models. According to the optimum slope angles predicted in this article and using the optimum slope angles achieved by other researchers at locations out of Iran but in the same range of latitudes, the correlations are obtained. The results showed that the optimum azimuth angle is zero for receiving maximum solar energy, so the maximum daily or monthly solar energy is independent of the azimuth angle. In addition, the results of the monthly optimum slope angles achieved using these correlations show more accurate results when compared with reference data.