The distribution of organic carbon fractions in soil aggregates in soil layer of 0～40 cm was studied under alfalfa（Medicago sativa L.）plantation of different ages.The result showed that soil organic carbon firstly increased and then decreased with the increase of alfalfa planting years,and it was the highest at 0～10 cm soil depth for all years,showing a phenomenon of surface accumulation.The soil granule structure of alfalfa plantation was dominated by＜0.106 mm aggregation which approximately accounted for 40%～90% of all aggregates,and decreased with the soil depth；＞0.25 mm aggregation was the highest at 4 years,which was firstly increased with planting years then decreased.The distribution of soil organic carbon in soil aggregates was mainly found in＜0.106 mm fraction of aggregates in the soil,and increased with the decreasing of particle size.The distribution of organic carbon in soil aggregates at 20～40 cm depth was higher than 0～20 cm depth,and increased with planting years then decreased.