Abstract: The flapping airfoil is a new type of energy harvester inspired by the flapping motion of birds. This paper aims to explore the potential of power-extraction enhancement of a flapping airfoil by using an oscillating trailing-edge flap, which invariably deflects towards the pressure side of the airfoil during the flapping cycle. Two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations at a Reynolds number Re = 4.7×10⁵ are conducted to compare the power-extraction performance of the proposed model with that of a plain flapping airfoil. In addition, the effect of airfoil thickness on the power-extraction efficiency is also analyzed. The results show that the heaving force, which can be increased during the entire flapping cycle by the use of trailing-edge flap, is the dominant contributor to the total power output of a flapping airfoil. It is also found that a better synchronization between the heaving motion and heaving force can be achieved by the use of a flap, yielding a greater power-extraction efficiency. The trailing-edge flap is proved to be beneficial to the power generation, especially at higher frequencies. The maximum power-extraction efficiency of the flapping airfoil with a trailing-edge flap can be increased by approximately 23.5% compared to the plain airfoil. Moreover, the leading-edge vortex will be suppressed by thicker airfoils, and there is an optimal airfoil thickness for the power extraction.