A finite element method based on ALE formulation has been adopted to examine the aerodynamic characteristics of an oscillating circular cylinder when the separated shear layers around thecylinder is stimulated by periodic velocity excitation on the cylinder surface. As a result, the negative damping caused by the unsteady lift force is found to be reduced by some cases of the periodic excitation, thereby stabilizing the cylinder aerodynamically. Furthermore, it is shown that the change of the lift phase relative to the cylinder displacement by the periodic excitation seems to play an important role in stabilizing the cylinder.