Abiotic stresses such as air pollution can limit plant productivity and survival. Over the past few decades, this pollution has become a serious problem in Seoul, Korea. In particular, O3 concentrations continue to rise as a direct consequence of human activity. The Ministry of Environment, Korea, has reported that 126 ozone warnings were issued nationwide during 2005, compared with only 48 in 2003 and 52 in 2000. The object of this study was to identify how trees, especially stomata showed their own defensive mechanism under ozone stress. Two-year old Liriodendron tulipifera, Cornus officinalis, Ginkgo biloba, Zelkova serrata, and Acer palmatum were selected for ozone fumigation in the phytotron(26±1.0°C/day, 22±1.0°C/night, 70±10.0%RH) under gradually increasing ozone concentration from 50ppb to 130ppb. We measured photosynthetic rates and water use efficiency for each one’s basic physiological status, and analyzed stomata responses specifically with anatomical and morphological changes of them under ozone treatment by scanning electron microscopy(SEM). The most remarkable species under ozone treatment was Liriodendron tulipifera, which showed that epidermal cells around stomata swelled and pressed stomata to be closed. Liriodendron tulipifera and Zelkova serrata showed decreasing numbers and increasing widths of stomata while Cornus officinalis and Acer palmatum showed increasing numbers and decreasing widths of stomata in saplings under ozone treatment. Ginkgo biloba didn't show changing numbers and morphological characteristics. Water use efficiency in control was higher than that in ozone treatment except Cornus officinalis.