Even　if　closing　windows,　atmospheric　PM2.5　can　still　penetrate　into　indoor　through　gaps　of　windows,　then　causing　the　indoor　environment　polluted.　In　order　to　evaluate　the　influence　of　windows　with　different　airtightness　levels　on　infiltration　characteristic　of　PM2.5,　a　longitudinal　monitoring　regarding　indoor　and　outdoor　PM2.5　mass　concentrations　and　meteorological　parameters　have　been　carried　out　in　two　offices　located　in　Beijing,　China　from　1st　July　to　2nd　November,　2014.　Research　results　showed　that　when　windows　and　mechanical　ventilation　were　closed　and　there　were　no　obvious　indoor　particle　sources,　the　indoor　PM2.5　mass　concentration,　the　I/O　ratio　and　the　air　exchange　rate　causing　of　window　gaps　of　the　sample　site　with　lower　airtightness　window　were　higher　than　the　one　with　higher　window　airtightness.　Furthermore,　the　I/O　ratio　and　air　exchange　rate　were　both　positively　correlated　with　outdoor　wind　speed,　while　negatively　correlated　with　outdoor　relative　humidity.　The　findings　can　provide　meaningful　reference　to　further　research　on　influence　of　atmospheric　PM2.5　on　indoor　environment　and　help　further　develop　approaches　to　control　outdoor　originated　indoor　PM2.5.　(C)　2015　The　Authors.　Published　by　Elsevier　Ltd.