In　this　study,　the　regional　transport　and　characteristics　of　PM2.5　pollution　were　examined　through　a　case　study　in　Beijing,　China.　The　results　from　an　intensive　monitoring　program　indicate　that　the　inorganic　particles　(sulfate,　nitrate,　and　ammonium),　organic　carbons,　and　the　elements　accounted　for　35.5,　24.2,　and　15.3%　of　the　total　PM2.5　on　an　annual　average　basis,　respectively.　The　proportions　of　such　PM2.5　components　also　showed　clear　seasonal　variations.　An　integrated　MM5-CMAQ　modeling　system　was　then　developed　to　examine　the　regional　transport　of　PM2.5　and　its　components　in　Beijing　within　four　typical　months　of　2010.　The　results　indicate　that　the　annual　average　total　trans-boundary　contribution　ratio　(TBCR)　is　42.2,　46.3,　77.4,　and　61.6%　for　the　concentrations　of　PM2.5,　sulfate,　nitrate,　and　ammonium,　respectively.　A　logarithmic　relationship　was　found　between　the　total　TBCR　and　the　PM2.5　concentrations　in　Beijing　for　different　seasons.　Further　investigations　showed　that　trans-boundary　transport　played　a　major　role　in　Beijing＇s　PM2.5　concentrations　during　the　period　of　high　pollution　levels,　with　an　annual　average　TBCR　of　54.6%.　As　a　result,　the　control　of　PM2.5　pollution　in　Beijing　needs　effective　cooperation　between　Beijing　and　its　surrounding　regions,　especially　during　periods　of　heavy　pollution.