This study carried out to investigate activities of the superoxide dismutase(SOD), peroxidase(POD) and diterpene cyclase of rice plant treated by environmental stress conditions. The rice leaves of Ilpum was shown to have the most highest specific activity of POD. POD activity in the susceptible cv. Ilpum and Hwasung during the disease progress show a increase, while for a decreasing activity in the resistant cv. Dasan. SOD activity of rice plants treated by chitosan were increased as the treatment concentration increased. At the chitosan application, the SOD activity in the susceptible cv. Hwansung at the rice leaves 14days after treatment was higher in treatment than chitosan-untreated control. The activity of POD in the susceptible cv. Ilpum decreased as long as UV-B irradiation, while that in the resistant cv. Dasan increased. Futhermore, activity of that in Dasan were similar to as time on UV irradiation, Hwasung showed two times higher than untreated control at 40 minutes after UV-B exposure. The activity of diterpene cyclase in rice plants inoculated with Pyricularia oryzae has much higher tendency than UV-B irradiation. The activity of diterpene cyclase in the rice leaves gradually increased until 5 days after inoculation with Pyricularia oryzae.