[1] The seasonal variation of boundary layer ozone over East Asia is investigated using a regional scale chemical transport model, with the initial and boundary conditions of chemical species obtained from a global chemistry model. Comparisons with observations indicate that the model reproduces the main daily and seasonal features of ozone over eastern Asia. The seasonal variation of ozone has a summer maximum and winter minimum at Waliguan station (on the northeastern edge of the Tibetan Plateau), rather than the spring maximum and summer minimum seen at many other observational sites in the East Asian Pacific rim region. Model results suggest that there is band of a high ozone between 35°N～45°N from the western boundary (70°E) to 130°E in the summertime. It is concluded that the seasonal transitions associated with the Asian monsoon system and transport from eastern/central China, Central/South Asia and even Europe are significantly responsible for the distinct ozone seasonal cycle at Waliguan.