Abstract: A clarification of several concepts is presented: the angle of repose, natural slope, surface friction angle, internal friction angle, and contact friction angle of sediment particles. This is based on a review of typical experimental methods for determining the angle of repose and its major influencing factors, along with the factors of the angle of surface friction. We present a discussion on the effects of sorting coefficient η, kurtosis coefficient K and skewness coefficient S on the angle of repose of non-uniform sediment, derive a relationship of the angle of surface friction versus the relative hidden degree of sediment particles from the concept of sediment incipient motion, and analyze the difference between the angle of repose for particles under the flows and the dynamic angle of repose. Finally, we suggest some ideas about the experimental tests and applications related to non-uniform sediment.