Hydrothermal barite is a typical low-temperature mineral formed during the mixing of hydrothermal fluid and seawater. Because of its extremely low solubility, barite behaves as a close system after crystallization and preserves the geochemical finger-print of hydrothermal fluid. In this study, the elemental contents and Sr isotope compositions of hydrothermal barites from the Yona-guni IV were determined using electron microprobe and LA-MC-ICP-MS respectively. On these bases, the fluid/sediment interaction during the hydrothermal circulation and physicochemical condition of barite crystallization were discussed. Results show that the 87Sr/86Sr values of hydrothermal barites from the Yonaguni IV are apparently higher than those of the seawater and associated vol-canic rocks, indicating the sufficient interaction between the hydrothermal fluid and overlying sediment. Monomineral Sr abundance shows large variations, reflecting the changes in barite growth rate during the fluid mixing. The mineralization condition in the Yo-naguni IV was unstable. During the crystallization of barite, hydrothermal fluid and seawater mixed in varying degrees, with the pro-portions of hydrothermal fluid varied from 36％ to 72％. The calculated crystallization temperatures range from 109 to 220℃. Sedi-ment plays a critical role during the mineralization process in the Yonaguni IV and incorporation of sediment component into hydro-thermal system was prior to barite crystallization and sulfide mineralization.