Abstract: In this paper, we discuss three fundamental problems of the theory of suspended sand particles based on the theory of fluid dynamics. They are:(l) the relative velocity of sand particles to the surrounding fluid in the turbulent flow,(2) the variation of the velocity fluctuation of the fluid when the suspended sand particles exist,(3) the profile of the vertical distribution of the concentration of the suspended sand particles in two-dimensional uniform steady channel flow and the modification of usual diffusion theory. The results of our theoretical analysis are somewhat different from the corresponding expressions-in the textbooks of hydraulics and river mechanics.