Abstract: Rill erosion is of great significance in hill-slope erosion. Soil erodibility for rill erosion is a very important parameter for soil erosion prediction with Water Erosion Prediction Project(WEPP) model. The estimation of the soil erodibility needed by WEPP was based on the potential detachment rate of rill erosion, measured with long rill channel. The dynamical erosion processes along a rill caused estimation error of the erodibility. Theoretical algorithm was given for determining the estimation error of the present method. Theoretical analysis indicated that for a well-defined rill channel, the estimation error of erodibility was mainly from the error of potential detachment estimation. The ideal estimated value of the potential detachment rate was the derivative value, at the initial position, of the function between sediment load and rill length. Computated results showed that the error in erodibility presently used in WEPP model was as high as 90% with the lowest value being higher than 50%. Therefore, it was suggested that both the method and the erodibility data for the WEPP model need to be reexamined and reevaluated. Possible ways for reduction of estimation error of the parameter were suggested.