In this work, root locus technique is used to study the active damping effects of a flexible structure governed by Euler-Bernoulli model including both bending and torsion motions. Two force actuators and three moment actuators are located at the beam tip (antenna), together with five rate sensors. It is found that with sufficiently large gain, at most, the vibration corresponding to the first five frequencies (ω 1 ω 5 ) can be completely eliminated, and none of the other frequencies can possibly be eliminated. These five frequencies correspond to two bending modes in the x-z plane, two bending modes in the y-z plane and one torsion mode. The root locus indicates that while active damping is effective only to low frequency vibrations, it excites all other modes even if only one mode is involved initially. Finally, the sensor noise rejection phenomenon is studied and it is found that the beam tip torsion motion completely rejects any zero-average periodic disturbance with period 2Lc, where L is the beam length and c is certain constant depending upon the beam material.