On the basis of meteorological fields provided by a mesoscale nonhydrostatic model (MM5), a three-dimensional regional chemical model (RADM) is applied to China to address the photochemical reaction mechanisms of surface ozone, with respect to ozone precursors such as NOx and NMHC. The behaviors of surface ozone in the Tibetan Plateau are much different than that in eastern China. Physical factors rather than photochemical factors may play important roles in determining surface ozone. Several model experiments are conducted to focus on these questions. Some results can be obtained as follows: (1) In polluted areas, the surface ozone is primarily dependent on its precursors (NOx, NMHC, etc.) and has highly nonlinear sensitivities to their perturbation. However, in clean areas photochemistry is not the controlling factor. Surface ozone on the Tibetan Plateau is sensitive to ozone perturbation in upper layers, suggesting that ozone above the planetary boundary layer may be a greater influence upon surface ozone. (2) The variations in OH and HO2 caused by perturbations in the precursors are very complicated, and the feedback effect of ozone is very important. There is a nearly linear relationship between percentage differences in HO2 and O3 for NMHC emission reduction, and a similar linear relationship also appears between percentage differences in OH and O3 for NOx emission reduction. (3) During summer midday time, the ratio of O3 to NO2/NO has a constant value. ¿ 1999 American Geophysical Union