Abstract: Two different pretreatment methods were used to measure grain size of surface sediments collected from the Southern South China Sea, and the influence of the biogenic opal on the grain size results were evaluated. The grain size results show that all the grain size fractions, grain size parameters, and the sediment classification were significantly influenced by the existence of biogenic opal. Based on the volume percentage of siliceous bioclastic, to construct the grain size distribution pattern of the terrestrial sediments on the Nansha Continental Slope, it is necessary to remove biogenic opal from samples in pretreatment. Comparison between the grain size results of two pretreatment methods suggest that the biogenic opal mainly occur in the range of 8 to 90 micron.