The mechanism of stomatal closing in response to O_3 was indirectly investigated by using H_2O_2 which is the intermediate product of O_3 metabolites. Stomata in epidermal strips close in response to H_2O_2. The effect of H_2O_2 on stomatal closing was dependent on the concentration of H_2O_2. 10 ppm H_2O_2 showed a clear effect on stomatal closing and 1000 ppm H_2O_2 induced complete stomatal closing after the treatment of 3 hours. Stomatal closing by H_2O_2 in intact leaf was also observed by measuring the diffusion resistance with porometer. It was found that the stomatal closing by H_2O_2 was not mediated by Ca^2+, and that was a different result observed in stomatal closing by water stress. Reversely, Ca^2+, showed a great inhibition on stomatal closing. The leakage of K^+ in epidermal strips was doubled in response to H_2O_2 when it was campared to the control. 10 ppm H_2O_2 decreased photosynthetic activity. Fv/Fm representing the activity of Photosystem Ⅱ was reduced about 4 % in 10 ppm H_20_2 and 8 % in 100 ppm H_2O_2 in the treatment of 1.5 hour. However, stomatal closing by 10 ppm H_2O_2 was reduced about 56 %. Accordingly, it can be suggetsted that stomatal closing by H_2O_2 is related with the decrease of photosynthetic activity, but it was chiefly induced by the change of the membrane permeability.