UV-B radiation affects the oil chemical composition as well as morphology, total protein and total phenol content of peppermint (Mentha piperita). The essential oil content was slightly increased by UV-B radiation, and the menthol content was significantly decreased because of increased synthesis of menthone, menthofuran and menthyl acetate. The effect on plant morphology was a significant inhibition of stem elongation, accompanied by changes in leaf area. Protein content and total phenols were increased when plants were irradiated with UV-B, whereas no significant effects were observed for chlorophyll content. The economic and ecological importance of UV-B effect on peppermint essential oil is also discussed.