The purpose of this study was to assess the effect of long-term cultivation and water erosion on the soil organic carbon(OC) in particle-size fractions.The study site is located at Nihegou Watershed in the Southern Loess Plateau, China. The soil at this site is loess with loose and silty structure, and contains macropores. The results showed that the OC concentrations in sediments and in the particle-size fractions of sediments were higher than those in soils and in the particle-size fractions of soils. The OC concentration was highest in the clay particles and was lowest in the sand particles. Clay particles possessed higher OC enrichment ability than silt and sand particles. The proportions of OC in the silt fractions of soil and sediment were the highest(mean value of 53.87% and 58.48%, respectively), and the total proportion of OC in the clay and silt fractions accounted for 96% and 98% of the total OC in the soil and sediment, respectively. The loss of OC was highest in silt particles, with an average value of 0.16Mg ha-1 y-1, and was lowest in the sand(0.003 Mg ha-1y-1). This result suggests that the fine particle-size fraction in the removed sediment may be an important indicator to assess soil OC losses. The purpose of this study was to assess the effect of long-term cultivation and water erosion on the soil organic carbon (OC) in particle-size fractions. The study site is located at Nihegou Watershed in the Southern Loess Plateau, China. The soil at this site is loess with loose and silty structure, and contains macropores. The results showed that the OC concentrations in sediments and in the particle-size fractions of sediments were higher than those in soils and in the particle-size fractions of soils. The OC proport was highest in the clay particles and was lowest in the sand particles. Crates particles possessed higher OC enrichment ability than silt and sand particles. The proportions of OC in the silt fractions of soil and sediment were the highest (mean value of 53.87% and 58.48%, respectively), and the total proportion of OC in the clay and silt fractions accounted for 96% and 98% of the total OC in the soil and sediment, respectively. The loss of OC was highest in silt parti cles, with an average value of 0.16 Mg ha-1 y-1, and was lowest in the sand (0.003 Mg ha-1y-1). This result suggests that the fine particle-size fraction in the removed sediment may be important indicator to assess soil OC losses.