||A sloshing liquid damper (SLD), mounted rigidly to a structure, is used to damp the unwanted motion of various structures by allowing liquid to slosh in a tank. In the present study, an SLD was used to reduce the roll motion responses of a barge, one of the supporting structures of offshore wind turbines. To solve the problem of interactions between the barge and SLD tank, a WAMIT commercial numerical code based on potential theory was used. The numerical results were verified with experimental results, conducted in a two-dimensional wave tank. The numerical prediction agreed well with the experimental results except for the peak values at resonant frequency. If liquid is allowed to slosh in a tank, the response amplitude operator (RAO)’s curve of roll motion of the barge has double peaks at two different resonant periods, compared with one peak in the case of frozen liquid. The characteristics of the roll motion of the barge were investigated by changing the position, size, and filled liquid depth of the SLD tank. It was found that a properly deigned SLD can help to improve the efficiency of a floating offshore wind turbine, which is sensitive to the roll motion of the supporting structure.