So far little has been reported in the literature on variation of carbon distribution in soil aggregates along a soil profile of a farmland converted from upland into paddy field at the Red Soil Critical Zone Observatory (RSCZO) in Yingtan, Jiangxi. This study was oriented to investigate (i) particle size composition and distribution of soil aggregates; (ii) changes in carbon content in soil aggregates, relative to fraction in particle size; and (iii) how iron-aluminum oxides affect carbon in the soil profiles of upland peanut fields, and old and new paddy fields of red soil. Soil samples were collected by layer from bottom to top in the profiles (0~120 cm) of upland peanut fields, new paddy fields (< 30 years) and old paddy fields (> 200 years) at the RSCZO located at the Sunjia watershed, Yingtan, Jiangxi Province of China. Soil aggregates in the soil samples were were collected and fractionated, by > 250 μm, 250~53 μm, and < 53 μm, kwith the wet sieving method. Carbon contents in the soil and aggregates were analyzed, and the relationships between soil iron-aluminum oxides and carbon in the soil aggregates were also discussed. Results show that the aggregates of the fraction of 250~53 μm accounted for 41.9%~58.2% of the total.in the soil profile, in all the three types of the field. The proportion of the fraction of > 250 μm, 250~53 μm and < 53 μm varied in a trend of "low - high - low", "high low low" and "low - low - high", respectively, in all soil layers with the land use pattern changing from upland peanut field to new and old paddy field. Carbon contents in the soil and soil aggregates increased significantly after the field was converted from upland to paddy, especially after long-term paddy cultivation. The formation of amorphous iron oxide in the soil affected significantly the content of total carbon in the soil of the upland peanut field and old paddy field. The content of soil total carbon in the soil profiles of all the fields decreased rapidly at first and then leveled off with soil depth, and in deep soil layers, the fraction of aggregates < 53 μm contributed the most to the increment in total carbon in the old paddy field.