In　order　to　evaluate　the　impact　of　windows　with　different　structure　characteristics　on　the　building　facade＇s　barrier　property　of　PM2.5,　a　longitudinal　monitor　regarding　PM2.5　mass　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　PM2.5　infiltration　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　0.7~1.0　μm　was　proportional　to　the　outdoor　PM2.5　mass　concentration,　while　the　proportion　of　particles　between　0.3~0.5　μm　was　inversely　proportional　to　the　outdoor　PM2.5　mass　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　Fin　showed　to　be　affected　by　outdoor　wind　velocity　and　relative　humidity.　The　Δp　and　Fin　of　sampling　site　1　were　always　lower　than　that　of　sampling　site　2.　©　2016,　Beijing　University　of　Technology.　All　right　reserved.