A world-wide attention has been paid to the air quality during the Beijing Olympic Games, especially the air pollution during the haze days. In order to monitor the air quality during Olympic Games, a project of Knowledge Innovation Program of Chinese Academy of Sciences, named as ‘Union Action Plan for Air Quality Monitoring and Forecasting During Olympic Game in Beijing and its Surround Area’, was carried out in Beijing in 2008. Different techniques have been used, including the ground-based [EQUATION]-[EQUATION] measurements, ground-based remote sensing and satellite measurements. The results demonstrated that the haze distribution on a regional scale can be derived from satellite measurements, and the relative humidity of 68.84% during the haze days in Beijing is only 4.23% larger than that during the non-haze days. Measurements from the Super Station at the Institute of Remote Sensing Applications (IRSA) revealed that the visibilities were about 5 km during the haze days, and the haze aerosol optical depths were mostly more than 1.0 during last August. The ground-based measurements from Beijing and its surrounding stations show that the averaged PM[EQUATION] and PM[EQUATION] mass densities were 68.08 [EQUATION]g/m[EQUATION] and 178.81 [EQUATION]g/m[EQUATION] during the haze days respectively, while the corresponding values are 29.58 [EQUATION]g/m[EQUATION] and 76.05 [EQUATION]g/m[EQUATION] during the non-haze days. The measured total column densities of NO[EQUATION] are far below the national standard, however, the space-borne observation by OMI/Aura showed high values in several areas in North China, including Beijing, Tianjin, Tangshan and some regions located in Shandong, Hebei, Shanxi and Henan provinces. Further analysis to the monitoring data and the backward track trajectory of wind profile indicated that the formation of haze in Beijing is mostly resulted from the wet growth of fine solution particulates under the stable atmospheric conditions with a plenty of water vapor, consequently leading to the rapid decrease of the visibility. As an evidence, a rapid increase of the PM[EQUATION] mass extinction across section was noticed that when the relative humidity is up to 95% from the measurements at the Super Station.