Since Bauer and the authors presented the paper showing that the sloshing of water in a tank acts as dynamic damper for reducing vibration, a lot of investigations have been carried out by civil engineers. Tuned sloshing damper (TSD) is not only simple and free of maintenance, but also available for reducing vibration. The authors showed that measured damper effect was in good agreement with one calculated by the authors' theory, in which the sloshing mechanism was simulated mass-spring systems. This paper deals with experimental investigation into the characteristics and the damper effect of TSD. The authors carried out systematically model test by using the TSD models and the tower models. The authors obtained the conclusions that the natural frequency of TSD increases as the amplitude of excited vibration but wave height response of water in TSD decreases, and the improved theory gives theoretical coupled natural frequency and response in good agreement with the measured one.