Abstract: The mineral composition of the bulk samples of the surface sediment in the eastern area off the Ryukyu Islands is studied with X-ray diffraction(XRD). It is found that the sediment consisting of smectite, illite+mica, kaolinite+chlorite, quartz, potash feldspar, plagioclase and calcite, is dominated by quartz, illite+mica, calcite and plagioclase, and kaolinite+chlorite and smectite occupy the second place with potash feldspar occasionally observed. According to the mineral composition, it is deduced that the sediment came from terrigenous, volcanic and biogenic sources.