In the present study, a new hydrokinetic energy harvester concept based on the tandem oscillating hydrofoils configuration is numerically investigated. Unsteady turbulent two dimensional flow simulations were performed, by using the commercial finite volume computational fluid dynamics code FLUENT, to study the power extraction from tandem hydrofoils oscillating inside a convergent duct. A sinusoidal function was used to interpret the plunging/pitching motion of NACA 0015 hydrofoil. Computations were carried out for the different configurations of area ratios (AR) that is defined as the ratio of the upstream hydrofoil’s inlet cross section (A1) to the downstream hydrofoil’s inlet cross section (A2) ranging from 1.0 (without the convergent section) to AR = 2.6. The study revealed that the power extraction was enhanced by the increased incoming flow velocity enabled by the geometry of the convergent duct, which positively affected the total pressure as well as the hydrofoil-vortex interaction time and resulted the generation of larger vertical hydrodynamic force. Upon analysis of the obtained results, it is found that the energy extraction efficiency is improved, compared to that of the conventional tandem configuration where the hydrofoils move in an open flow space, and, as the area ratio (AR) increased, the amount of extracted power becomes higher in a proportion ranging from 4% to 80%. The concept was numerically tested for tandem and single configurations and for non-dimensional frequencies in the range of f∗=0.04 - 0.20. One interesting finding is that, due to some positive effect of hydrofoil-vortex interaction, and at a specified area ratio and frequency, the downstream hydrofoil efficiency exceeds the efficiency of a single hydrofoil located at exactly same position as the downstream hydrofoil in the tandem configuration. The frequency and the area ratio are found to be critical factors that determine whether the downstream hydrofoil-vortex interaction is favorable or unfavorable. An optimal area ratio/frequency combination results in a positive hydrofoil-vortex interaction effect and allows the downstream hydrofoil to contribute positively to the total power extraction as a single hydrofoil. Based on the present results, and for an optimal efficient energy harvester design, a new correlation is suggested in order to properly control the area ratio in accordance with the variation of the oscillating frequency leading to more power production and high performance of the oscillating tandem hydrofoils. It is anticipated that this new innovative technique will be applied to improve the existing energy harvester devices.