The investigations on the organic carbon (OC) of core sediments were carried out in Chongming easttidalflat(CM ) during Scirpus mariqueter growing stage (from Aprilto December 2004)in Yangtze Estuary.The Yangtze Riverannually transports a runoffdischarge of30,000 m3/s, carrying about480 million tons ofsediments to the estuarine and coastalarea,which formed a great OC pool.In the sampling spots,seven quadratsof50 cm × 50 cm and five sedimentscoresof20 cm deep (40 cm deep in December)were randomly established in orderto collectvegetations and core sediments samples during the low tide each month except November.After pretreatment,the core sediments were sieved and theirOC contents were measured according to the potassium dichromate method. The results show that the higher surface sediment OC content in summer comes from allochthonous terrigenous particle settlements on the Chongming eastmiddle tidalflatS.mariqueter zone.In autumn and winter,the decomposing of the defoliated S.mariqueter increases the surface sediments OC content.Settling velocity,sedimenttemperature and S.mariquetergrowth are the main factorsthatcan controlthe sedimentcarbon content.Summeristhe “carbon losing”period ofthe tidal flatsediments,while from September,itchanges into the “carbon accumulating” period ofsediment OC poolbecause ofthe decomposing ofdead S.mariquetercommunity in the sediments.From this alternation of“carbon losing”period and “carbon accumulating”period,weconcludethatcarbon in the OC pool of the middle tidalflatS.mariqueter zone sediments mainly comes from the atmospheric carbon rooted by S.mariqueterphotosynthesis. The investigations on the organic carbon (OC) of core sediments were carried out in Chongming easttidalflat (CM) during Scirpus mariqueter growing stage (from Aprilto December 2004) in Yangtze Estuary. The Yangtze Riverannually transports a runoff discharge of 30,000 m3 / s, carrying about480 million tons of sediments to the estuarine and coastalarea, which formed a great OC pool. in the sampling spots, seven quadrats of 50 cm × 50 cm and five sedimentscoresof 20 cm deep (40 cm deep in December) were randomly established in orderto collectvegetations and core sediments samples during the low tide each month except November. After pretreatment, the core sediments were sieved and their OC contents were measured according to the potassium dichromate method. The results show that the higher surface sediment OC content in summer comes from allochthonous terrigenous particle settlements on the Chongming eastmiddle tidalflatS.mariqueter zone. In autumn and winter, the decomposing of the defoliated S.mariqueter increases the surface sediments OC content.Settling velocity, sediment temperature and S.mariqueterrowth are the main factorsThatcan control the sedimentcarbon content.Summeristhe “carbon losing ” period of the tidal flatsediments, while from September, itchanges into the “carbon accumulating ” period of sediment OC poolbecause of the decomposing ofdead S.mariquetercommunity in the sediments. From this alternation of “carbon losing ” period and “carbon accumulating ” period, weconcludethatcarbon in the OC pool of the middle tidalflatS.mariqueter zone sediments mainly comes from the atmospheric carbon rooted by S.mariqueterphotosynthesis.