Abstract: Along with the development of construction technology, impervious system for medium-low RCC dams has transferred from two-graded concrete to three-graded concrete, and even no impervious layer. However, the width of impervious layer is not determined clearly. Based on the study of permeability coefficient of RCC dams, we carry out seepage calculation and seepage stability analysis for 100m-level RCC dam of full-face three-graded aggregate. The results show that the application of C20 three-graded aggregate RCC at upstream impervious area of 100m-level RCC dam could satisfy the requirements of structure seepage stability. Further, the recommended width of impervious layer is provided.