Decrease of ozon layer thickness and increase of ultraviolet radiations are some of the biological environmental problems emanated from human activities. High ultraviolet radiation B (280-320 nm , UV-B ) affects plants harmfully. Fennel is one of the precious medicinal plant, widely used in pharmacy, perfume, cosmetic and hygienic as well as food industries. The seeds or fruits of this plant have so much essential oils. Medicinal properties of these plant are attributed to this. essential oils. This research studies the anatomical structure, essential oils quality and quantity changes in fennel from umbeliferae family, all affected by high ultraviolet tradiation, emanated from a 40- watt lamp in the field conditions in three phases: before flowering, at flowering and after the formation of seed. Anatomical changes of treated plants radiated during all the phasses are including: inc easing of thickness of cuticular layer on adaxial epidermis in petiole, decreasing of palisade parenchyma tisssue (decrease in size and height of the cells). Increasing of surface of perivascular collenchyma tissue, and decreasing of their depth. Reduction in diameter of secretory canals and increase in thickness of stem selerenchyma cells. The amount of essential oils in leaf, stem, flower and seeds decreased in the most of phases, and essential oils components changed under ultravioler radiations. The amount of trans nethole. The most important compound of this essential oil, has offen decreased but Estragol, Fenchone and Limonene showed an increase, in most cases. The above results indicated the sensitivity of fennel to ultraviolet radiations.