This paper is concerned with the active vibration control of a free-free beam. The beam is reduced to a finite-degree-of-freedom system by the modal analysis, in which the mode function is derived from the transfer matrix method. An active control force is produced by a pair of tendons and a DC servo motor attached to the beam. The state of the beam is presumed by the minimal order state observer and the control force is determined by the digital optimum regulator theory. It is found that the active tendon control method is effective for suppressing the vibration of the free-free beam.