In　this　study,　a　coupled　MM5-CAMx　air　quality　modeling　system　was　proposed　for　simulating　the　SO2　concentration　in　Beijing,　China　during　the　heating　season.　The　Particulate　Source　Apportionment　Technology　(PSAT)　was　introduced　into　the　modeling　system　to　investigate　SO2　source　apportionment　from　regions　nearby　Beijing　and　regions　in　Beijing.　The　results　indicated　that　the　influence　from　regions　outside　of　Beijing　including　Hebei,　Tianjin　and　Shanxi　could　not　be　ignored,　and　the　total　Emission　Source　Contribution　Ratio　(ESCR)　from　these　regions　was　10.8%.　Metropolitan　Beijing,　including　Chaoyang,　Fengtai,　the　city　center　of　Beijing,　Haidian,　and　Shijingshan,　with　an　ESCR　of　82.5%,　was　the　main　contributing　area　in　Beijing.　The　results　obtained　could　provide　useful　information　for　Beijing＇s　SO2-emission　control　and　abatement.　This　method　could　effectively　identify　SO2　pollution　problem　in　a　metropolis　region　and　thus　provide　a　scientific　basis　for　air　pollution　control　and　mitigation.