Tuned Liquid Damper (TLD) utilizing the motion of shallow liquid for absorbing anddissipating the vibrational energy is studied with emphasis on liquid motion. Amathematical model based on the nonlinear shallow water wave theory is presented todescribe the liquid motion in a rectangular tank. Liquid damping is evaluated semi-analytically and is included in the formulation. Mechanical properties of: TLD are alsoexperimentally investigated using the shaking table. It is found that the liquid motion inTLD is strongly nonlinear and reveals a hardening-spring property even under smallexcitation, Good agreements between the simulation and the experimental results areshown when no breaking wave occurs. The model presented in this study is expected toserve as a tool for TLD design.