Abstract: The incipient motion of cohesive sediment and the mixture of cohesive sediment and coarse particle are researched systematically. On the basis of turbulent flow theory, a formula for determining the critical shear stress of cohesive sediment with the Bingham stress of cohesive sedi ment is ascertained. According to a series of experiments, the conditions of incipient motion for the mixture of cohesive sediment and coarse particle in different cases are discussed and formalized. The result of all above-mentioned formulae are in agreement with experiment data. In addition, a qualitative analysis about the conditions of incipient motion of cohesive sediment and the mixture of cohesive sediment, and coarse particles above which dense flow exista are carried out.