Distribution and fractions of organic carbon (C) in sediment profiles of 3 types of wetland ecosystems (peat fen, humus marsh and marshy meadow) were analyzed. Data show that the sediment profiles of the wetlands can be divided into an organic C depositing horizon (the upper layers) and a mineralilluvial horizon (the lower layers) based on the characters in the distribution of organic C and its compositions in the profiles. The organic C depositing horizon of peat fen, humus marsh and marshy meadow had a depth of 110, 60, 15 cm and contained about 295, 280, 60 g/kg (dry matter) of organic C on an average, respectively. Light fraction of organic C ([WTBX]P[WT]<1.7) accounted for up to 70% of the total organic C in the organic C depositing horizon of peat fen, humus marsh, but only to 16% in the same horizon of marshy meadow. For the 3 types of wetland ecosystems, the contents of organic C in the mineralilluvial horizon were all smaller than 30 g/kg, which was little fractionated as the light fraction (<2%). It was also shown that for the layers of wetland sediment profiles with a content of organic C above 180 g/kg, there was a close correlation existing between total organic C and the content of the light fraction, and below that level, the content of light fraction was small (0.2~17 g/kg) and also showed little response to changes in total organic C. This indicated that the large presence of the light fraction determined the accumulation of organic C in the organic C depositing horizon of the wetland sediment profiles. Thus, all of the results presented suggest that the extent to which organic C decomposed ([WTBX]e.g[WT]., as indicated by the percentage of light fraction as total organic C) be the primary factor to determine the accumulation of organic C in the sediment profile of wetland ecosystems.  A procedure was developed to estimate the stock of organic C accumulated in the sediment profile of wetland ecosystems. It was estimated that the stock of organic C was 6.62×104, 4.90×104, 1.44×104 t/km in the upper 1 m depth, and 8.16×104, 6.81×104, 2.24×104 t/km in 2 m in the sediment profile of peat fen, humus marsh, and marshy meadow, respectively. 