In rotating machinery, the unbalance of the rotor causes many resonances at the critical speeds corresponding to various modes. In this paper, a method of vibration control for rotor systems utilizing the disturbance observer (D.O.) is proposed. The nonlinear terms, unbalance, parameter variations and uncertain terms of a rotor system are lumped into a disturbance term, and this term is cancelled by using D. O.. As a result, the vibrations of the system are controlled to a small amplitude in all over the rotational speed range. Using the information of disturbance obtained by D. O., the unbalance of the first mode is estimated simultaneously. Moreover, the effects of some parameter errors of the control system are also investigated. The validity of the proposed method is verified through numerical simulations and experiments.