The　Pearl　River　Delta　(　PRD)　is　one　of　the　areas　that　are　facing　a　serious　fine　particulate　matter　(　PM2.5)　pollution　in　China.　In　order　to　grasp　the　variation　law　of　PM2.5　concentrations　in　the　atmospheric　environment　and　its　impact　on　the　indoor　environment,　this　research　has　taken　a　certain　naturally　ventilated　office　building　as　the　monitoring　object　to　constantly　monitor　its　mass　concentration　of　indoor　and　outdoor　PM2.5　and　meteorological　parameters　from　February　to　July　2014　and　from　December　2014　to　January　2015　in　Guangzhou.　During　the　test　period,　the　external　windows　and　mechanical　ventilation　system　stayed　closed　and　there　were　no　other　indoor　pollution　sources.　The　survey　results　showed:　(　1)　among　all　diurnal　variations,　the　diurnal　variation　in　hourly　average　mass　concentrations　for　outdoor　PM2.5　showed　that　the　night　experienced　greater　variation　than　the　day;　in　terms　of　seasonal　variation,　Spring　experienced　the　most　severe　outdoor　PM2.5　mass　pollution　(　115　mu　g/m(3)),　followed　by　Winter　(　95　mu　g/m3);　(　2)　the　indoor　PM2.5　mass　concentrations　were　highly　correlated　with　that　outdoor,　meanwhile　the　indoor/outdoor　PM2.5　mass　concentrations　ratios　(　I/O　ratios)　were　in　a　broad　range;　(　3)　both　outdoor　PM2.5　mass　concentrations　and　infiltration　factors　were　significantly　correlated　with　meteorological　parameters　(　temperature,　wind　speed　and　relative　humidity);　(　4)　compared　with　the　related　research　conducted　on　Beijing,　PM2.5　concentrations　and　infiltration　factors　in　both　the　two　regions　were　affected　by　wind　speed　and　relative　humidity,　but　the　impact　of　temperature　on　outdoor　PM2.5　concentrations　was　not　discernible　in　Beijing.　The　research　results　can　serve　as　a　reference　for　analyzing　the　influence　rule　of　outdoor　PM2.5　situation　to　the　indoor　environment　in　the　Pearl　River　Delta　region　and　its　control　technology.　(C)　2015　Published　by　Elsevier　Ltd.