Some concerns are arising due to the decreasing levels of stratospheric ozone as a result of industrial activities having destructive effects on biosphere through increasing levels of ultraviolet radiation. In order to evaluate the effects of UV-B radiation on sesame, a field experiment was conducted in the Research Farm Isfahan University of Technology in 2007. The experiment was arranged as factorial in a randomized complete block design with four replications. The experimental factors were UV-B at three levels (UV-B less light, natural light and UV-B enriched light) and three sesame genotypes (Varamin, Oltan and Yekta). The number of days to flowering and maturity, plant height, leaf dry weight, total dry matter, number of capsule per plant, grain yield, harvest index and the leaf chlorophyll content were decreased though the specific leaf weight was increased as a result of the enhanced level of UV-B radiation. Filtering of the UV-B radiation led to an increase in the number of days to flowerimg and maturity, plant height, leaf area index, number of leaves per plant and the number of seeds per capsule. However, the number of capsule per plant, grain yield, harvest index and the leaf chlorophyll content were decreased under UV-B filtered light. There were significant differences among genotypes for the number of days to flowering and maturity, plant height, leaf area index, leaf dry weight, number of leaves per plant, shoot dry matter at flowering, number of capsule per plant, number of seeds per capsule, seed weight, seed oil percentage and the leaf chlorophyll content. With genotypes tested here, Varamin proved superior in terms of the number of days to flowering, number of days to maturity, leaf area index, leaf dry weight, shoot dry matter, number of capsules per plant, grain oil percentage and the grain yield. The interactions between the radiation levels and genotype were significant for the number of days to maturity, number of capsules per plant, the number of seeds per capsule and seed oil percentage. The number of days to maturity in varamin and the number of seeds per capsule in yakta under UV-B filtered light and the number of capsules per plant and the seed oil percentage under natural light were superior in Varamin. The results of this study indicate that the increased level of UV-B as an environmental stress may negatively affect the growth and development of sesame leading to a decrease in grain yield of this important oil seed crop. در تاریخ 14/8/1387 توسط کمیته تخصصی زیر مورد بررسی و تصویب نهایی قرار گرفت. 1- استاد راهنمای پایان نامه دکتر مرتضی زاهدی 2- استاد مشاور پایان نامه دکتر پرویز احسان زاده