The flow-induced vibration of a circular cylinder is simulated numerically. The finite element method is used to solve the three-dimensional incompressible Navier-Stokes equations, and the flow field is coupled with the cylinder motion. The vortex field around the cylinder is visualized under the alternative vortex condition. The vortex field is shown to have three-dimensional structures in the wake of the cylinder. It is found that the frequency of vortex variation increases both in space and time by the flow-induced vibration.