The　window　cracks　is　one　of　the　important　passageway　that　atmospheric　fine　particulate　matter　(PM2.5)　get　through　into　the　indoor　environment　and　then　causing　the　indoor　personal　PM2.5　exposure.　In　order　to　evaluate　the　characteristics　of　air　infiltration　through　the　window　cracks,　a　longitudinal　monitoring　regarding　both　indoor　and　outdoor　PM2.5　mass　concentrations　and　meteorological　parameters　(e.g.　temperature,　wind　speed　and　relative　humidity)　were　carried　out　in　an　unoccupied　office　located　in　Beijing　from　September　2013　to　August　2014,　with　the　condition　of　no　mechanical　ventilation　and　no　indoor　PM2.5　pollution　sources.　Additionally,　a　mathematical　model　of　air　infiltration　rate　based　on　a　large　number　of　measured　data　combined　with　mass　conservation　theorem　and　the　method　of　mathematical　statistics　was　developed.　The　results　show　that　the　outdoor　weather　conditions　and　atmospheric　PM2.5　concentrations　is　the　important　factor　of　the　air　infiltration　rate　in　a　room　which　has　a　certain　window　structure　(the　PM2.5　penetration　factor　(P)　is　0.93±0.01　and　deposition　rate　(k)　is　0.10±0.03　for　the　sampling　site).　Influenced　by　the　change　of　the　outdoor　wind　speed,　the　average　air　infiltration　rate　is　about　0.10　h-1　at　static　stability　days,　and　it　is　about　0.22　h-1　at　gentle　breeze　days,　as　well　as　0.39　h-1　at　moderate　breeze　days.　©　2017,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.