Abstract: RAMS-CMAQ, a coupled regional air quality modeling system, is used to analyze the temporal and spatia1 variations of the surface O3 concentration over Beijing and its surrounding areas in August 2008． The result indicates that the model can generally reproduce the main observational feature of O3 and the wind，and the pollution control can reduce the O3 concentration noticeably. On the other hand, analysis of the formation of high O3 concentration indicates that the O3 concentration would be high under the favorable weather condition, for example, on both 2 August and 24 August, when there is convergence in a weak wind field or the southerly wind transports air pollutions. The impact of the pollution control in August is better in the surrounding areas than in the urban districts of Beijing.