The　objective　of　this　study　was　to　investigate　the　influence　of　atmospheric　fine　particulate　matter　(PM2.5)　pollution　on　indoor　environment.　Time　series　of　indoor　and　outdoor　PM2.5　mass　concentrations　in　a　naturally　ventilated　building　were　monitored　continuously　and　simultaneously　in　winter　in　Beijing　when　haze-fog　episodes　happen　frequently.　Relationships　between　indoor　and　outdoor　PM2.5　mass　concentrations　were　examined　and　discussed.　Indoor　and　outdoor　PM2.5　mass　concentrations　showed　close　correlation　(R-2=0.9104)　when　the　windows　are　closed　and　there　are　no　obvious　indoor　particle　sources.　The　peak　values　of　average　indoor　PM2.5　mass　concentrations　appeared　at　night,　which　were　1.5　times　higher　than　the　minimum　values　in　daytime　and　showed　an　increasing　trend　from　Monday　to　Friday　in　workdays.　The　indoor/outdoor　PM2.5　mass　concentrations　ratios　(I/O　ratios)　are　closely　correlated　with　outdoor　wind　speed　and　relative　humidity.　A　prediction　model　was　developed　based　on　a　method　of　mathematical　statistics　and　it　was　applied　to　estimation　of　indoor　PM2.5　mass　concentrations　by　using　outdoor　PM2.5　mass　concentrations　and　meteorological　parameters　as　inputs.　There　is　a　good　agreement　between　model　results　and　the　measured　values　with　correlation　coefficients　of　R-2　=　0.95.　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　in　the　haze-fog　episodes.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.