Solar arrays are one of the most promising renewable energy technologies for energizing future building. For roof top solar panels installation, knowledge of the optimum tilt angle is important to have the maximum annual or seasonal energy yield. this is due to the fact that variation in tilt angle affects the amount of solar radiation reaching the panel surface. In this study ,a mathematical model and data obtained from Typical Meteorological Year (that calculated of radiation data for 12 years (1994-2005) on horizontal surfaces in Karaj) is used to estimated the total (global) solar radiation on a tilt surface and to determine the optimum tilt angle for a solar panel in karaj , iran. The best orientation for solar arrays in Karaj is due south. It is found that the optimum tilt angle change between (May, June, July and August) and (January) throughout the year.in winter(December, January and February) the tilt should be ,in spiring (March, April and May) , in summer (June, July and August) and in autumn (September, October and November) .the annual optimum tilt angle dependent on many factors such as the latitude of the location and weather condition. the yearly average of this value was found to be and this would be the optimum fixed tilt throughout the year