Abstract: The decrease of visibility in at mosphere is the prominent problem of urban environment in the world.The recentyear's monitoring results in Beijing indicate that the pollution level of fine particles shows an increasing trend,which is a major factor to be responsible for visibility decreasing through the extinction.From the analysis of Beijing four season's samples of fine particles obtained in 2001,the mass concentration of fine particles in Beijing went beyond the USEPA standard badly and the annual average value was 6 times higher than the Standard.The relationship bet ween concentration of fine particles and weather conditions were analyzed qualitatively and quantitatively,based on the simultaneous observational data of fine particles and the weather conditions in this research.It was found that the mass concentration of fine particles was relevant notonly to the environmental condition but also to the meteorological conditions closely.There was a significant negative-correlation between mass concentration on fine particle and wind speed in Beijing with correlation coefficient -0.89 in winter breeze day and -0.60 in spring breeze day.Relative humidity was a key factor to influence mass concentration of fine particles.Mass concentration of fine particles was showed to be highly positive correlated with relative humidity,which correlation coefficient was near 1.0 in summer,0.99 in spring breeze day,and 0.81 in fog day in fall.Precipitation had great washout effecton mass concentration of fine particles.After heavy rainfal in summer,the mass concentration of fine particles decreased quickly.The concentration of fine particles was much lower than that in drizzle day.The results also showed that drizzle had some washout effects to remove fine particles.The synoptic regime had relatively great in fluence upon the mass concentration of fine particles.Before the synoptic regime coming,the fine particles pollution was seriously.When the regime passed,the PM 2.5 mass concentration decreased quickly and dramatically under north wind covering.