An active mass damper has been developed to reduce the vibration occured in ships especially in the superstructures. Active mass damper was designed so as to reduce effectively the vibration of fluctuating frequency and amplitude induced by several vibration sources such as main engine and propeller as well as the steady vibration of single frequency and amplitude. Vibration characteristics were confirmed by the experiment with the aid of model structure. Then the active mass damper was applied to 150, 000 DWT tanker to verify the efficiency of reducing vibrations, and obtained satisfactory results. In this paper, (1) The active mass damper reduced the vibration of the superstructure of the 150, 000 DWT tanker especially in the resonance peak area. The vibration due to unbalanced moment decreased to one sixth of its original level, vibration derived from propeller exciting force to one half and vibration induced by main engine exciting force to one fifth, simultaneously. (2) The active mass damper can be applied to not only the vibration of superstructure of ship but local vibrations of various part.