Abstract: Land use change such as conversion of cropland to forest strongly affects the soil organic carbon(SOC)sequestration.The objective of this paper was to assess the dynamics mechanism of SOC sequestration in protection forest in Three Gorges reservoir area by means of organic matter fractionation and stable C isotopes(δ¹³C)technology. The soil of three land use patterns(LA. a permanent Pinus massoniana forestland, LB. a deciduous Sycamore maple forestland was planted on continuous maize field in 2002, LC. a continuous maize field)were sampled. Soil samples were separated into three aggregate size classes. And inter-vs.intra-aggregate particulate organic matter were isolated into eight soil aggregate fractionations by density method. All fractions were analyzed for their SOC content and δ¹³C. Results showed that long-term agricultural used land significantly decreased SOC content by 57.2%and 32.2% in the top 10 cm and in the ≥10-30 cm depth layer, respectively. But after 11 years afforestation, the total amount of soil C increased by 34.5% and 42.9% in the 0-10 cm and in the ≥10-30 cm depth layer, respectively. Forest-derived carbon(new C)contributed 48% and 52% to the total SOC storage in the afforested systems in the 0-10 cm and ≥10-30 cm depths, respectively. The stored SOC before 2002(old C)only lost by 9.2%in LB. Afforestation could significantly increase new SOC fixed and improve the old SOC physically protected.