Ultraviolet B (UV-B) radiation had deleterious effects on plants. The purpose of this study was to investigate the effect of UV B irradiation time period on genetic variation and quantity and quality of essential oils of Ocimum Basilicum L.. Seeds were exposed to 0.38 W/m2 of UV-B radiation for 0, 7, 15, 30 days (ten minutes each day). The GC-MS results showed UVB treatments brought about distinct alternations in quality of essential oils of treated plants. Estragol, Eucalyptol, Caryophyllene, Bisabolene, Cubebene and Methyleugenol were identified as major of the compounds. The results of Ward clustering based on essential oils indicated that the mid- time irradiation treatment had the highest similarity to the essential oil composition with control plants. To evaluate the genetic variation in treated plants, 6 ISSR primers were used. The molecular markers used indicated that the 7-day control and treatment samples were significantly separated from the 15 and 30-day treatments of the basil plant samples by UV irradiation, respectively. The UPGMA dendrogram and plots from PCoA analyzes separated the different treatment groups. So that the 7-day control and control samples were significantly separated from the 15 and 30-day UV treated of the basil plant samples, respectively. It can be finally concluded that the UV-B treatment can significantly affect the genetic materials beside of the essential oils content of basil, especially in the long time exposures. پرونده مقاله