Abstract: In order to evaluate the impact of windows with different structure characteristics on the building facade's barrier property of PM_2.5,a longitudinal monitor regarding PM_2.5mass concentration and air particle size distribution was conducted in two offices located in Beijing,China. The two offices were equipped with windows with different structure characteristics,and both offices were monitored between July and October,2014. In this paper,the characteristics of PM_2.5infiltration of the two types of windows were compared and potential influential factors were identified,and the infiltration pressure Δp was proposed and the relation among windows structure characteristics,wind speed and relative humidity was analyzed. The survey results showed that the proportion of outdoor air particles between 0. 7 ~ 1. 0μm was proportional to the outdoor PM_2.5mass concentration,while the proportion of particles between 0. 3 ~ 0. 5 μm was inversely proportional to the outdoor PM_2.5mass concentration. The barrier property of both window types on particles smaller than 0. 7 μm was low. However,the windows with larger airtightness can show higher barrier property. From the study,the infiltration pressure Δp and infiltration factor F_in showed to be affected by outdoor wind velocity and relative humidity. The Δp and F_in of sampling site 1 were always lower than that of sampling site 2.