Top soil structure changes and their effects on erosion were studied under simulated rainfall in red soils derived from shale, Quaternary clay and granite. The results showed as followed: Soil crust developed easily in the red soil derived from shale, which accelerated runoff generation and made runoff velocity and sediment content rise to peak value quickly; while in the soil derived from Quaternary clay, because of high aggregate stability, crust developed more difficultly and was easy broken down. Companied with the crust formation and breakdown, the runoff velocity and sediment content rose slowly and fluctuated in a small range. Crust did not found in the soil derived from granite, on which surface sandy layer appeared during erosion process. The sediment size distribution depended on aggregate stability and runoff transportation. The percent of fine particles (<0.02 mm) in sediment were higher than that in soils in the soils derived from shale and granite. And the sediment of soil derived from Quaternary red clay contain much more coarser component which were compound aggregates.