In this paper, the effects of nondimensional frequency f*, pitch amplitude (0), and airfoil thickness on the energy extraction performance of an oscillating wing wind power generator were numerically investigated. It is found that the optimum value of f* or theta(0) exists to achieve the maximum energy efficiency. Additionally, the thickness of airfoil also significantly affects the efficiency and the flow patterns around the oscillating foil. For thin airfoils, a relatively large-scale vortex was normally generated at its leading edge. This vortex detached from leading edge might be able to be caught by the airfoil again and then reutilized to increase its work capacity. By contrast, no induced leading edge vortex is formed on the upper surface of a thick airfoil. Nevertheless, the pressure difference between the upper and lower surface of the oscillating thick airfoil is greater than that of thin airfoil. Thus, the portion of the output power contributed by the oscillatory heaving motion is greatly increased and high energy extraction efficiency can still be achieved. For airfoils with moderate thickness, both flow phenomena observed on thin and thick oscillating airfoils that have high wind energy utilization efficiency are all likely to occur, depending on the adopted motion parameters.