The relationship between the concentration of 137 Cs and the grain size of soil was analyzed. 137 Cs concentrations
in clay layer and sand layer in a dam land were also compared. The results show the higher concentration of 137 Cs, the smaller the soil grain. The mechanical composition of the sediment at the outlet of the
watershed has significant difference compared with that of soil in woodland and cropping slope fields, which indicates that there is soil particle selectivity occurred in the course of erosion, but not in the process of sediment transmitting. All these mean that the results need to be revised if 137 Cs in sediment of the outlet is used to estimate erosion modulus of the watershed. 137 Cs concentration in sediment in dam land was analyzed and the result shows that there is a close relationship among 137 Cs concentration, sediment thickness and erosion intensity of per rainfall, erosion types. Applying the characteristic of radionuclide in sediment samples, it is possible to solve the proportion relationship between slope erosion and gully erosion at watershed scale.