Abstract: An experiment was conducted to examine the variations of the physiological characters in leaves of Monarda didyma L. exposed to different ozone concentrations (40 (CK), 80, 120 and 160 nmol·mol^-1) in open top chambers (OTC_s). The results were as follows: (1) An obviously visible leaf injury was observed under 120 and 160 nmol·mol^-1 O₃ with fumigation of 14 days, and a large number of yellowbrown patches appeared on the leaves, while no visible injury was observed under 80 nmol·mol^-1 O_³. (2) After 14-days fumigation, elevated O₃ (160 nmol·mol^-1) decreased net photosynthetic rate (P_n), stomatal conductance (G_s), PSⅡ electron transfer quantum yield (Φ_PSⅡ) and PSⅡ maximum photochemical efficient (F_v/F_m) by 30.2%, 53.1%, 25.7% and 3.6%, respectively. (3) Elevated O₃ (160 nmol·mol^-1) increased malondialdehyde (MDA) content (74.6%) and the production rate of superoxide radical (O₂^-_·) (31.0%) after fumigation of 14 days, while significantly deceased superoxide dismutase (SOD) activity (19.5%) and soluble protein content (20.9%), indicating that elevated O₃ led to serious lipid peroxidation and oxidative stress and destroyed the function of antioxidant system in plants. The content of abscisic acid (ABA) significantly increased (31.0%), suggesting the acceleration of the aging and abscission of plant leaves (the proportion of yellow leaves increased). Consequently, O₃ pollution may have adverse effects on the growth of Monarda didyma and its edible and medicinal values.