PM2.5　is　the　major　pollutant　in　most　cities　of　China,　especially　contributed　significantly　to　the　poor　air　quality　in　Beijing.　This　study　aimed　to　investigate　the　long-term　trend　(2000-2015)　of　PM2.5,　based　on　intensive　observation　and　comprehensive　literature　investigation　of　PM2.5　and　its　chemical　components.　Results　showed　that　the　annual　average　concentration　of　PM2.5　generally　decreased　by　1.5　mu　g　m(-3)　year(-1)　from　2000　to　2015　under　the　implementation　of　16　phases＇　air　pollution　control　measures.　In　the　most　polluted　season　(winter),　four　change　stages　were　found　related　with　emission　control　effect　and　meteorological　conditions:　PM2.5　decreased　in　2000-2008　and　2010-2013,　increased　in　2008-2010,　and　was　at　a　high　level　in　2013-2015.　As　for　detailed　chemical　components,　OC,　soil　dust　and　typical　elements　(Si,　Ca,　Fe,　Mn,　Cu,　Pb　and　K)　presented　a　downward　trend　generally.　EC　had　almost　no　change　before　2003,　increased　from　2003　to　2007,　but　decreased　after　2007;　this　may　be　caused　by　the　replacement　of　fossil　fuel　and　control　of　biomass　emission.　The　continuous　rising　of　OC/EC　and　SOC/OC　(secondary　organic　carbon,　SOC)　in　recent　years　illustrated　the　secondary　carbonaceous　species　pollution　is　becoming　serious.　SO42-　showed　a　slight　increase　from　2000-2013,　but　decreased　obviously　from　2013-2015.　NO3-　and　NH4+　had　an　upward　trend　during　the　past　decade.　The　proportion　of　secondary　inorganic　aerosol　(SIA)　increased　at　a　rate　of　0.7%　yr(-1),　and　it　has　become　the　major　composition　of　PM2.5　in　Beijing　instead　of　carbonaceous　component　since　2009.　In　general,　PM2.5　change　trend　indicated　the　emission　mitigation　measures　implemented　in　Beijing　have　reduced　the　primary　PM2.5　effectively.　However,　the　control　of　secondary　components　should　be　paid　special　attention　in　order　to　further　improve　the　air　quality　in　Beijing　effectively.