Grain size analysis and multivariate statistics have been carried out for the 416 surface sediments collected from the northwestern South China Sea. There are 14 types of sediments from gravel to clay, dominated by the sediment of 4φ~9φ upto 71%. The sediments vary between 0.99φ~7.44φ in mean grain size, 0.54~3.72 in the coefficient of sorting, -0.32~0.78 in skewness and 0.62~3.92 in kurtosis following the formulas by Folk et al.. R-mode cluster analysis shows that there are two clusters. The first cluster is composed of the sediments of -2φ~5φ, and the second cluster consists of the sediments of 6φ~11φ. The total eigenvalue of the first 5 factors is up to 89.38%, with the factor 1,2 and 3 as the main. The factor 1 is composed of the positive load of 9φ~11φ and negative load of 3φ~4φ. The factor 2 consists of positive load of 6φ~8φ and the factor 3 of negative load of -2φ~0φ. Coarse sediments, which are well sorted, are mainly distributed on the continent shelf with strong hydrodynamics and multiple sources. To the contrary, fine sediments, which are usually poorly sorted, often lie in low energy environments, such as the gulf, low energy continental shelf, continental slope and deep basin with a simple source.