In recent years, some process-based soil erosion prediction models are proposed in order to promote soil loss prediction on slope. In such prediction models,estimation of hydrodynamic parameters in soil erosion processes is the key to predict soil loss on slope exactly. Based on the runoff experiments in laboratory, themechanism of rill erosion and sediment transport in rill are studied and the method to estimate the妙draulics of flow in rill, such as critical shear stress for rill initiation,mean velocity of flow in the rill, Manning roughness coefficient, soil erodibility for grill erosion, as well as rill section characteristic are discussed in this paper.