< p > A solar collector is required to absorb solar radiation and transfer the absorbed energy into a heat transfer fluid with a minimum of heat loss. In assessing the performance of a collector, it is therefore important not only to determine its ability to absorb solar radiation but also to characterize its heat losses. The ability of a collector to absorb solar radiation is largely determined by its optical and geometric properties. One of the important parameters that affect the performance of a solar collector is its tilt angle with the horizontal. This is due to the fact that the variation in tilt angle affects the amount of solar radiation reaching the collector surface. In this study, a mathematical model is used to estimate the total (global) solar radiation on a tilted surface and to determine the optimum tilt angle for a solar collector in Izmir, Turkey. Total solar radiation on the solar collector surface with an optimum tilt angle is computed for specific periods. It is found that the optimum tilt angle changes between 0 degrees (June) and 61 degrees (December) throughout the year. In winter (December, January, and February) the tilt should be 55.7 degrees, in spring (March, April, and May) 18.3 degrees, in summer (June, July, and August) 4.3 degrees, and in autumn (September, October, and November) 43 degrees. The yearly average of this value was found to be 30.3 degrees and this would be the optimum fixed tilt throughout the year.