Abstract: From the statistics of environmental observations, it shows that the air quality in Shanghai during the 2010 World Expo (1 May to 31 Oct) is the best for the same period since 2001. The number with good air quality days adds up to 181, accounting for 98.4% of the total 184 days. From the integrated data of the near-surface meteorological observation in Shanghai in recent 10 years, NCEP/NCAR reanalysis data from 1981, and the circulation indices of National Climate Center of CMA since 1951, the phenomenon is preliminary investigated, revealing the near-surface meteorological situation and the atmospheric circulation characteristics which affect the dispersion and deposition of air pollution during the 2010 World Expo and the same period in history. The detailed mechanisms for the impact of atmospheric condition on air quality are further investigated, and the major results are as follows.The greatly enhanced easterly winds, less calm wind days, significantly less near-surface temperature inversion and more precipitation favor the dispersion and deposition of air particles and the improvement of air quality in Shanghai during the 2010 World Expo.Subtropical high over the western Pacific during the 2010 World Expo is more intensive and extensive, and extending further west compared to that of recent years. As a result, the easterly airflow to the south of subtropical high is strengthened which leads to the prevailing easterly winds in Shanghai.During the 2010 World Expo in Shanghai under the same situation of various dominant wind, e.g., calm wind, NNE-SSE wind, or NNW-SSW wind, corresponding API indices are lower than the value of the same period in 2001—2009, which reflects the effect of pollution reduction due to "Joint Implementation of Air Quality Regulating and Controlling over the Yangtze River Delta Region during the World Expo" carried out by Jiangsu, Zhejiang and Shanghai.Although, with the joint action and its advantage, it is noticed that there are three heavy pollution episodes occurred in Shanghai during the World Expo due to the depressed weather conditions of transportation and dispersion. This result depicts that weather situation plays one of the major roles which decide the good air quality during the World Expo in Shanghai.