The abundance of radiolarian, diatom and sponge spicule and H4SiO4 in pore-waters increase abruptly at the boundary between Early and Late Oligocene (about 30-27.5 Ma) at Site 1148 of the northern South China Sea (SCS), indicating high biogenic silica accumulation during this time. At the same time (about 30-28 Ma), high biogenic silica deposition occurred in the cen tral equatorial Pacific. Comparison of the biogenic silica accumulation at Site 1148 of the SCS with that at Site 929 of the Atlantic verifies that the biogenic silica accumulation between the low lati tude Pacific and Atlantic oceans expresses the evident relationship of compensation during theOligocene. Biogenic silica accumulation decreased in the Atlantic, whereas it increased in the Pa cific at the boundary between the Early and Late Oligocene. It resulted from the formation and presence of North Atlantic deep water (NADW) in the Atlantic basin, indicating an intensive basin-basin fractionation. XRD analysis and SEM observation of the samples from Site 1148 demon strate that most of radiolarian, diatom and sponge spicule have suffered from dissolution and reprecipitation, suggested by the opal-A→opal-CT transformation. As a result of the transformation,porosity increased, but dry and bulk densities decreased, reflecting the consequence of diagenesis on the physical property of sediment.