Abstract: The field measurements showed that the ozone concentrations over Yangtze Delta Region were highest in spring and high ozone concentrations were detected most frequently during spring time.The aims of this study are to reproduce the physical and chemical processes over Yangtze Delta Region using a regional air quality model,and explain the formation of the high concentration ozone in spring from physical and chemical aspects.The USEPA’s community multiscale air quality (CMAQ) was applied to the Yangtze Delta Region,and one case of abnormal high ozone concentration in May 2000 studied.The model generally reproduces the temporal and spatial variations of ozone observed at the ground stations of the region.The causes of the high concentration observed at Sheshan,Jiaxing and Lin’an on May 11,2000 were analyzed.Modeling results indicate that meteorological fields have extremely significant effects on the distribution of the regional air pollutions.It is also found that with proper effects of wind field,the pollution source in Shanghai region could make a significant influence on the air quality of the Yangtze Delta Region.