Abstract:The red soil hilly region, located in the tropical and subtropical zones of Southeast China, consists of 9 provinces and 1 autonomous region. The area of region is about 1.13 million km2, and hold 30% of the population. Because it has plentiful light, heat and water resources, the red soil hilly region has a high potential for agricultural and economic development. However, soil erosion and seasonal drought are major constraint for sustainable agriculture due to improper land use and uneven distribution of rainfall. Efficient rainfall use and erosion control, therefore, are very advantageous from eco-environmental and agricultural perspectives. Soil aggregate stability has substantial effects on soil porosity and, therefore, may influence infiltration rate and runoff processes under rainfall. The objects of this study were to investigate the relationship between aggregate size and soil aggregate stability and determine the effect of aggregate size on runoff and soil loss. In this study, three red soils derived from Quaternary clay were studied. Aggregate size ＜2, 2 to 3, 3 to 5mm of each soil were exposed to simulated rainfall with an intensity of 60mm h-1. The Le Bissonnais’ method was applied to simulate the breakdown mechanisms of slaking in fast wetting, and mechanical breakdown by wet stirring. The aggregate stability index, normalized mean weight diameter (NMWD), indicated that fast wetting caused the most severe disruption to aggregates. The NMWD in the fast wetting treatment ranked in the order of aggregate size ＜2mm, 2mm to 3mm, and 3mm to 5mm. There was a significant interaction between soil loss, runoff rate and aggregate size. The results showed that as clod size increased, the runoff rate decreased, ranging from 0.36 to 0.62mm min-1. The small aggregate size is likely to form a seal, consequently, the soil loss increased with decreasing aggregate size. The largest MWD of sediment materials were found in aggregate size ＜2mm of each soil.