This paper presents the results of study on a tuned liquid damper (TLD). The TLD uses air and liquid inside a U-type tank as the mass of dynamic damper of a structure. By using the Lagrange method, the dynamic equation of the structure with this U-type tuned liquid damper is derived. Vibration behaviors of the structure with and without a U-type TLD subjected to a simple harmonic excitation are examined in the time domain. Effects of a perforated baffle inside U-type tank on vibration reduction is also studied.