Abstract: A two-dimension air quality model has been developed to simulate the ozone concentration in the Yangtze River Delta. The simulated ozone concentrations are in general agreement with those of measurement. Process analysis is conducted for ozone formation in the area of Changshu of Jiangsu Province. Results show that dry deposition decreases the daily ozone concentration with a value of averaged 2.1 μL/(m³·h), horizontal transport and chemical production rate of ozone have obviously diurnal variations. However, transport process totally decreases the ozone concentration with a value of averaged 1.9 μL/(m³·h), while the chemical process has a net contribution to ozone formation with a value of averaged 5.3 μL/(m³·h), indicating the high oxidative capacity of regional atmosphere. A chemical production efficiency study was also conducted , the result indicated that each NO_x molecule can produce 7.5 O₃ molecules during its life from emission to removal.