X-ray diffraction and scanning electron microscopy analyses were performed on 200 samples taken from deep-sea Oligocene sediments from a core depth of 450 m and 850 m at ODP Site 1148 in the northern South China Sea. The results show that opal-A in siliceous fossils was transformed into opal-CT in the interval between 475 m and 600 m. With increasing depth, the fragmentation of siliceous fossils increased, the X-ray peaks of authigenic quartz became more prominent, and the X-ray peaks of opal-CT reduced in intensity, indicating the occurrence of opal-CT in quartz transformation in this interval.