Abstract: The suspended sediment usually occupies a large portion of the total load transported by streams in large fluvial rivers. Therefore, it is of crucial importance to study the mechanism involved in the suspension of suspended sediment. In this paper, we derive an equation for suspended sediment transportation and the sediment diffusion coeffcient based on the two-fluid model by introducing the concept of dispersion velocity, and apply the theory to calculate the concentration profiles of uniform two-dimensional open channel flows. Comparisons with the experimental data from Einstein and Chien and classical diffusion models are also presented, which show that present theory agrees well with the experimental data. In addition, we analyze the variations of influences of different effects contributing to the suspended sediment in the vertical direction. It is found that the suspension of the sediment in open channels is the result of three different actions, which are the turbulence diffusion of mixture, particle turbulence, and particle collisional stress. The result also shows that it is insu cient to ascribe the sediment in suspension solely to the turbulence of the flow.