In this paper, a two-phase flow model is presented which simulates the fluid and sediment motions in the sheet flow regime on a flat bed under oscillatory flow conditions. The model is developed based on the continuity equations and linearised momentum equations for the fluid and sediment phases, respectively. All major forcing terms such as the intergranular stresses and the turbulent stress are included in the model. From the detailed computations and comparison with the available laboratory data it has been demonstrated that the model is capable of predicting fairly accurately both flow kinematics and sediment concentrations. In particular, the model predicts that the well known phenomenon of fluid velocity over-shoot that exists in clear water also appears in the case of lighter sediments but vanishes when the materials are heavier, which is in perfect accord with the experimental observations considered. (C) 1999 Elsevier Science B.V, All rights reserved.