The mechanism of the growth enhancement effect of plants induced by a low-pressure oxygen plasma is investigated. The growth enhancement of plants would be caused by the regulation of the antioxidative activity in plants. Antioxidative activities in seeds and stems of radish sprouts are varied by a low-pressure oxygen plasma irradiation. The thiol compounds amount in seeds of radish sprouts, which relates to the antioxidative activity, has the same tendency as the active oxygen amount in a low-pressure oxygen plasma. Major active oxygen species controlling the antioxidative activity and the thiol compounds amount in radish sprouts would be the singlet excited oxygen molecule, which is revealed by the light emission intensity at 762 nm.