Fine　particulate　matter,　generally　known　as　PM　2.5　,　has　great　impact　on　air　quality　and　human　health.　Although　closing　external　windows　can　help　prevent　outdoor　PM　2.5　form　going　indoors,　many　studies　have　shown　that　a　significant　number　of　particles　can　still　pass　through　the　building　façade　through　the　cracks　around　the　window.　In　order　to　quantify　the　influence　of　external　window　crack　characteristic　and　relevant　parameters　(such　as　room　dimension)　on　the　indoor　ρ(PM　2.5　),　a　longitudinal　study　monitoring　outdoor　ρ(PM　2.5　),　indoor　ρ(PM　2.5　)　and　important　outdoor　meteorological　parameters　was　carried　out　in　six　offices　located　in　the　Dongcheng　and　Chaoyang　District　in　Beijing,　China.　In　addition,　a　model　was　developed　from　an　existing　model　developed　by　the　authors,　based　on　two-month　field-measured　data　from　five　unoccupied　offices　located　in　the　central　area　of　Beijing,　and　was　then　validated　against　a　new　dataset　measured　at　another　sampling　site.　In　this　model,　a　comprehensive　structure　characteristic　coefficient,　A　P　,　was　adopted　to　reflect　the　influence　of　external　window　crack　structure　and　room　dimension.　Further,　this　study　adopted　a　method　that　can　be　used　to　calculate　the　value　of　A　k　based　on　the　building＇s　external　window　crack　structure　and　room　dimension,　which　are　easier　to　be　obtained.　The　model　is　capable　of　quantifying　indoor　ρ(PM　2.5　)　based　on　instant　outdoor　ρ(PM　2.5　),　with　consideration　of　impact　from　external　window　crack　characteristics,　room　dimension　and　outdoor　meteorological　conditions,　i.e.　outdoor　wind　speed　and　relative　humidity.　The　model　was　validated　by　measurement　data.　The　prediction　results　showed　that　under　the　same　outdoor　conditions,　such　as　ρ(PM　2.5　)　and　temperature,　indoor　air　quality　for　ρ(PM　2.5　)was　significantly　affected　by　external　window　crack　structure.　The　I/O　ratio〔the　ratio　between　indoor　ρ(PM　2.5　)　and　outdoor　ρ(PM　2.5　)〕　was　generally　within　0.4　and　0.7.　Additionally,　when　increasing　the　window　crack　height　by　50%　or　the　window　crack　depth　by　50%,　the　corresponding　I/O　could　be　decreased　by　33.6%　and　31.9%　respectively.　The　study　shows　that　narrower　window　crack　height　and　deeper　window　crack　depth　with　higher　window　air-tightness　can　control　indoor　ρ(PM　2.5　)　well　under　the　condition　of　infiltration.　©　2017,　Editorial　Board,　Research　of　Environmental　Sciences.　All　right　reserved.