Mass　concentration,　seasonal　variation　and　sources　of　organic　carbon　(OC),　element　carbon　(EC),　methanol-soluble　organic　carbon　(MSOC),　and　seven　carbon　components　(OC1-4,　EC1-3)　were　detected　by　thermal-optical　analysis　of　353　PM2.5　samples　in　Xi＇an　in　2017.　The　results　show　that　the　average　mass　concentrations　of　OC,　EC,　and　MSOC　were　(17.56±11.83),　(4.08±2.95)　and　(11.10±6.77)　μg•m-3,　respectively.　The　seasonal　trend　of　the　OC　concentration　follows　the　order　winter＞spring＞summer＞autumn.　The　seasonal　trend　in　EC　concentration　follows　the　order　winter＞spring≈autumn＞summer.　The　average　MSOC/OC　value　is　0.64±0.20.　The　highest　value　is　observed　in　winter　and　the　lowest　in　summer.　The　correlation　between　OC　and　EC　is　good　in　spring　(r2=0.76),　but　the　correlation　is　poor　in　winter　(r2=0.43).　These　results　indicate　that　the　source　of　the　carbon　aerosols　was　different.　The　content　of　secondary　organic　aerosols　was　estimated　by　the　EC　tracing　method.　The　average　mass　concentration　of　SOC　accounted　for　51.9%,　38.4%,　37.3%　and　44.0%　of　OC　in　spring,　summer,　autumn,　and　winter,　respectively.　The　main　sources　of　carbonaceous　aerosols　were　analyzed　by　principal　component　analysis.　The　results　show　that　carbonaceous　aerosols　originate　mainly　from　coal　and　vehicle　emissions　in　Xi＇an.　©　2019,　Editorial　Office　of　FINE　CHEMICALS.　All　right　reserved.