Abstract: 3D numerical simulations of the wake flows behind axial-flow tidal current turbines have been carried out using a CFD software to study the interference between the wakes behind different impellers in different conditions of rotor installation, focusing on a single-rotor model without duct and a ducted dual-rotor model with a shaft spacing of 1.8D (where D is the blade diameter) for comparison. Results show that, the wake flow of single-rotor model was relatively stable with a velocity nearly the same as outside the wake, except for the lower velocity in a small wake region of less than 1D from the rotor. In the case of dual-rotor model, we observed a stronger interference between the two rotors, a lower velocity in the wakes, and a lower energy loss in the wakes than the single-rotor case. This leads to a considerable increasement in energy conversion ratio of the rotors. Physical model tests have been conducted and the measurements verified the main conclusions above and the effectiveness of our CFD method.