Abstract: Sediments from Dongsha area of the Soth China Sea are pretreated by different methods for grain size analysis and by integrated methods for particle size analysis in order to determine influences of carbonate and organic matters on the particle-size characteristics. The results showed that average grain sizes and components within 63~4 μm grain sizes reduced significantly after removal of organic matter, and changes in contents of organic matters and those in the average grain sizes were negatively correlated, indicating that organic matter in Dongsha sediments was deposited with suspended materials of fine grain and colloidal material, and had a clear role in flocculation of fine grains. Average grain size reduces significantly after removal of carbonate, but influences of carbonate on contents of components within grain size scope are not clear, and the relationship between changes in the carbonate content and those in average grain sizes is not close.