This paper presents a two-phase flow model that simulates the fluid and sediment motions in the sheet flow region under oscillatory conditions. Some major forcing terms such as the fluid/particle and particle/particle interactions and turbulent stresses are included in the model. By improving some assumptions of most existing models, the present model specifies the equivalent sand roughness and bed concentration as a function of the Shields parameter, which is variable with time and is physically more realistic over a mobile flat bed. A wave friction factor, which is governed by a new parameter, is obtained from the present model formulation. The present model is shown to provide a more accurate estimate of sediment concentrations than those models using a constant equivalent sand roughness. Numerical analyses also show that the suspended sediment retards the mean velocity and suppresses turbulence.