Abstract: Qiongzhou Strait is one of the largest straits in China. Dominanted by both the EW-trending and NW-trending structural system, the shoreline there is zig-zag in plan shape. Of the sediments on the Strait, sands and gravals, composed of quartz grains and basalt fragments, are distributed on the deep trough where current velocity is relatively high; the finer sediments are accumulated on low-energy areas, such as the bays along coasts and the outer edge of the west mouth of the Strait. A deep trough, 51 km in length and 2.7 km in width, is lying along the bottom of the Strait. On the both sides of the axis of the trough, there is a series of EW-trending depression and eminence features. In the mouths of the Strait, because of the ncreasing of cross-section areas, tidal currents spread and their velocity is reduced. Therefore, two tidal deltas, comprising shoals and troughs which arrange alternatively and radially, are respectively formed in the east mouth and west mouth. On the west delta, the sediments are clayey sands or sandy clays. Contrastly, the sediments on the east delta, being mainly medium or coarse sands, are coarser.