The influence of rotation on the behavior of a two-dimensional rotating cylinder in flow was investigated by using the discrete vortex method combined with the 4th order Runge-Kutta method. There are three stages for rotational speed dependence of hydrodynamic force on a fixed rotating cylinder, the stage where lift force increases and drag force decreases, the stage where lift and drag forces increase in proportion to the square of rotation ratio, and the stage where lift and drag forces increase in proportion to rotation ratio. As a fixed rotating cylinder begins to rotate and the rotational speed increases, the vortex train that causes VIV weakens and disappears. When the cylinder is mounted on a spring, the vibration frequency of the spring mounted rotating cylinder becomes lower. This indicates that the added mass of the rotating cylinder is increasing as the rotation ratio increases.