Abstract: Grain-size analysis is carried out for the 95 surface sediment samples from the southwestern part of the South China Sea(3°56'~11°53'N,108°30'~113°47'E), where clayey silt and silty clay dominate, with sand and silty sand only occurred in the most southwestern shallow part. There shows a trend of decreasing grain-size, increasing sorting and decreasing skewness from the shallow water to deeper water. The Kriging method was employed to insert data of the grain-size parameters (mean grain-size, sorting coefficient and skewness), and then the trend analysis was carried out for the interpolated data using the GSTA program. It is revealed that the seabed topography plays a key role in the sediment transport. There is a topographic high in the area including the Guang-ya submarine plateau, the Nan-wei-tan plateau, and the Yong-shu-bei submarine plateau. To the north of the high, sediments from the western South China Sea continental shelf are transported into the Southwest Basin of South China Sea; to the south of the boundary, however, materials from the Sunda shelf and the Kalimantan Island are transported into the Nan-wei-xi and Bei-kang slope basin passing by the shelf break. The episodic sedimentary events, such as the turbidity current, also have significant impacts on the grain-size distribution pattern. The aggregation of the components <8 μm should not be ignored. Further researches are required for details.