This study investigated the size distributions of aggregates and primary particles in the sediments eroded from five typical soils in China under natural rainfalls. The experiment was conducted on five runoff plots packed with the Red Soil

Purple Soil

Loess

Cinnamon Soil and Black Soil. Sediment samples in four runoff events of these plots under natural rainfall were collected during June to September in 2013 and 2014. Using the pipette method and wet sieving technique

the compositions and size distributions of primary particles and aggregates were measured. Moreover

the sediment aggregates with different sizes were sieved and determined for the size distribution of primary particles. The results showed that: (1) The sediment size distribution was affected by both soil texture and rainfall intensity. The aggregate size distribution was more suitable as a better indicator for sediment transport. (2) Larger portions of silt-sized (0.002～0.05 mm) and clay-sized (＜0.002 mm) aggregates were obtained in the sediments of five types of soils. (3) However

no obvious enrichment of primary particles was observed in the eroded sediments of any soil but the Red Soil

which possessed slightly elevated percentages of silt and clay in the corresponding sediments. (4) When further analyzing the primary particle-size composition of sediment aggregates with different size

the highest percentage of clay particles was determined in the silt-sized aggregates of the Red Soil. For the rest four types of soils

the mechanical composition of eroded sediment and the mechanical composition of the aggregates at all sizes were all similar to the source soil.