The availability of daylight, including diffused sky light and direct sunlight, in urban open spaces is an important environmental factor in designing and regulating buildings in high density urban areas. The establishment of a scientific and quantitative basis to evaluate the daylighting impact of a building has been one of the major concerns among daylighting and city planning communities. This study on daylight initially investigates methods to predict the daylight levels in urban environments. The methods to deal with complex urban geometries and their optical properties were explored. A computer-based illumination model for daylight in urban environment was developed adopting numerical methods. The direct components from the sky were estimated by the finite elemental area method, and the reflected components were approximated by the recursive iteration method. Using the model, analyses of daylight availability in the downtown Phoenix area were conducted . Urban zoning parameters were explored with respect to their potential to the impact on the daylight utilization in commercial buildings in the target area. A special emphasis is given on analyzing the performance of inter-reflected light in urban open spaces and in commercial building interior spaces.