We investigated the influence of the airfoil geometry on the performance of the plunging airfoil numerically. In the present study, two circular arc airfoils with the different thickness were used and the numerical calculations were carried out within the laminar flow region to obtain the data of the extremely small micro air vehicles. Moreover, the arbitrary Lagrangian-Eulerian method was adopted to solve the moving boundary problem. The calculation results are summarized as follows. (1) The thickness of the airfoil has a strong effect on the lift and thrust forces, and these forces for the thick airfoil are insensible to the change in the angle of attack compared with the ones for the thin airfoil. (2) The optimal oscillating frequency of the plunging airfoil has a strong relation to the conditions of the flying or swimming animals, birds, fish and insects.