The work is aimed to develop an active control method for suppressing unbalanced vibration of an elastic rotor system via the utilization of active bearings. The active bearing was implemented by the developed control system to actively control the displacements of the bearing for regulating the bearing forces. To address the issue of uncertainties associated with the unbalanced forces and internal dynamics of the rotor system, the Extended State Observer (ESO) was utilized to provide an accurate estimation of the generalized disturbance that consisted of the external disturbance in the form of radial forces caused by the unbalanced rotating mass of the rotor, and the disturbance associated with uncertainties in the internal dynamics of the rotor system. Active Disturbance Rejection Control (ADRC) method was utilized to cancel the effect of the generalized disturbance, allowing a simpler control input implementation to the active bearing. It was shown in this work that the ESO was able to accurately estimate the disturbances affecting the rotating system in real time. As the result, the active bearing control system could reject the disturbances effectively to minimize the unbalanced vibration of the rotor system.