Abstract: Numerical simulations based on the finite element method (FEM) are conducted to study the behaviour of flatfish cages under currents and waves. The pitch of the bottom frame of the cages and the maximum mooring line force are investigated. A series of tank physical model tests have also been performed to compare and verify the numerical results, and a satisfactory level of agreement has been achieved in the three tested wave and current conditions. Furthermore, the motion of flatfish cages under waves and currents can be visualized. It is found that the maximum pitch of the bottom frame of flatfish cages is 9° with a current speed of 26cm/s, and that under waves is 12°. The maximum pitch and the mooring line force are both greater under combined wave and current effects than when waves and currents are acting individually. When the direction of currents does not agree with that of waves, the maximum pitch decreases as the angel between the incident directions of currents and waves increases. Field measurement will be conducted to further validate the simulation results for improved accuracy of the numerical study.