A method of reducing the torsional vibration of a motor using a dynamic absorber has been studied. The vibration characteristics of a motor-mounted dynamic absorber have been analysed. Theoretical and experimental investigations show that the absorber is highly effective, even for the reduction of low-amplitude torsional vibrations where viscous dampers do not function as well. Furthermore, it is shown that the response of the control system does not deteriorate, even when an absorber is applied to the 2-phase servomotor of a servo system. Therefore the twice-line-frequency torsional vibration of a 2-phase servomotor, caused by an unbalanced voltage supply, can be reduced by the application of the dynamic absorber.