In an agroforestry system, short wave radiation distribution under the canopy of the trees is important to the activity of the annual crops growing beneath. A three-dimensional model is proposed to describe the short wave radiation distribution under the tree canopy in an agroforestry system. In the model, the agroforestry system is assumed to be planted in regular arrays and have spherical crowns. The short wave radiation that reaches a prescribed point on the ground is computed as the integration of radiation from all the differential directions of the whole hemisphere. The canopy depth in each differential direction is computed, and the extinction coefficient of the tree canopy is estimated by computer analysis of photograph images of the tree crowns taken by a conventional camera. A major advantage of this method is that the laborious leaf area index (LAI) and leaf inclination measurement can be eliminated. The model is able to predict both the short wave radiation distributions below the canopy of an agroforestry system at a prescribed time and the diurnal variation of the total hemispherical short wave radiation at a prescribed point below the canopy. Comparison of the modeled results with the measured values showed that the proposed model describes the daily patterns of the short wave radiation under the tree canopy quite well for both discontinuous canopy and overlapping canopy and for different shading conditions. The difference between the modeled daily total values of the short wave radiation under the canopy and the measured daily total values is usually less than 5% of the global radiation. Results from sensitivity analyses of the model to crown radius and canopy gap fraction are reported.