The aim of this research was to investigate physiological and biochemical responses of cowpea (Vigna unguiculata (L.) Walp) to ozone. There were two main factors of the experiment; level of ozone concentration at 40 and 70 ppb and plant ages at 7 and 21 days. Plants were grown in fumigation chambers in which inlet air was filtered by a charcoal filter. Additional ozone was given 8 hours/day for 7 days in ozone fumigating chambers. The ozone concentration in the control chambers was less than 10 ppb. The results showed the biomass of ozone-fumigated plants was significantly lower and leaf injury of ozone fumigated plants was significantly greater compared to the control group. The major visible-injury symptom appeared as chlorosis on the upper surface of the leaves. Antioxidant levels in the charcoal filtered (CF) plants and ozoned plants had significant differences because of their detoxification role in removing ozone and its derivatives. The ozone treatment of 7-day-old plants showed superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) levels significantly higher than in 21-day-old plants and total ascorbate concentrations significantly lower than 21-day-old plants. These results showed that different ozone concentrations exhibit different effects on antioxidant production. Analysis of antioxidants daily for 7 days found that antioxidant levels rapidly changed. Notably, SOD and total ascorbate could be selected as indicators for ozone-effect monitoring in plants. This indicates that cowpea is sensitive to ozone and may be usable as an ozone bioindicator. In conclusion, plant age, ozone concentration and the duration to exposure to ozone were the main physiological or biochemical responses of cowpea. An efficient defense system was generated from a combination of antioxidants.