Ground-level ozone (O₃) is considered as one of the most phytotoxic air pollutants due to its significant damaging effect to plants. Ozone concentration is rising at a rate of approximately 0.5%-2% per year over the mid-latitudes of the northern hemisphere due to the rapid industrialization and urbanization during the last three decades. High levels of O₃ occur during summertime in most parts of China, and are expected to increase further as NO_X precursors are rising. Open top chambers were used to explore the stomatal characteristics (i.e., stomatal density, aperture, and size) of six greening tree species (Populus deltoides ‘55/56’×P. deltoides ‘Imperial’, P. euramericana ‘74/76’, Fraxinus chinensis, Platanus orientalis, Robinia pseudoacacia, and Sophora japonica) in response to different O₃ exposure treatments for one growing season. One-year-old seedlings of the selected species, representing a wide range of woody species used for urban greening in Beijing, showed great variation in the stomatal characteristics among the species. Generally, stomatal density, stomatal aperture, and stomatal size were all significantly decreased with increasing ozone concentrations. Specifically, there were significant differences in stomatal characteristics among the six tree species. Furthermore, there were significant interaction effects among the tree species, ozone treatments, sampling times, and their interactions. All six species showed a significant negative linear correlation (P<0.05) between stomatal parameters and the O₃ dose index AOT40 (O₃ concentration increased hourly over a threshold of 40nmol/mol during daylight hours). Among the linear relationships between the stomatal parameters and AOT40, Populus showed the maximum slope, and the order of the O₃ sensitivity was Populus > P. orientalis > F. chinensis > R. pseudoacacia > S. japonica. Our results provide new evidence for research on the effects of ground-level O₃ pollution on leaf tissue structure and functions of greening plants under environmental change.