Abstract: This study presents results of a study on estimation of daylight availability on horizontal and south oriented vertical surfaces as well as energy saving in industrial parks of Tehran by daylighting. Beyond our natural affinity for daylight, it is much more effective than electric lighting at entraining the circadian system because the circadian system responds only to high levels of blue light, such as those found in daylight. Horizontal and vertical illuminances were calculated for three established stations by equations of Illuminating Engineering Society of North America. Synchronically illuminances were measured over 15 days between 12 July and 1 August 2007 to confirm calculated data. The correlation of measured and calculated values was reasonable (r = 0.703). Regression models were developed between measured and calculated values (r2 = 0.8). Horizontal and vertical illuminances were predicted for 11 industrial parks of Tehran during a working year (294 days) by fitted models. The minimum, maximum, mean and SD of predicted horizontal values found to be of 12.45, 108.12, 66.48 and 24.71 KLx, respectively. Considerable frequency of occurrence of horizontal illuminance in all places (9.7%) was related in values more than 100 KLx. Additionally it was revealed that in 55% of working year daylight could be sufficient for maintaining indoor standard illuminance of 500 Lx. Results of this study suggest high daylight availability and high potentiality of energy conservation in Iran.