Resulting from the shortage of fossil fuels, solar energy plays a more and more important role in power supply in future. The amount of radiation flux incident upon a solar collector is mainly affected by the installation angle. Proper design of a collector can increase the irradiation received. In this paper, the optimal angle in Taiwan is calculated according to three different radiation types, i.e. the extraterrestrial radiation, global radiation predicted by empirical model and ten-year observation data from 1990 to 1999. Some differences among them are analyzed considering the geographic and climatic factors. The results show that the angles calculated from the extraterrestrial and predicted radiations are simply latitude-dependent and thus can be well determined, but the angles estimated from observation data vary from location to location and are generally flatter than those from other two radiation types. It tells us that the collector must be installed with a flatter tilt angle when it works in a cloudy or pollutant environment.