Abstract: New ages of planktonic foraminiferal and nannofossil events across the Oligocene/Miocene boundary (also the Paleogene/Neogene boundary)are introduced.The 2004 international geologic time scale placed the boundary at the base of magnetic subchron C6Cn.2n, close to a period of weak eccentricity amplitude referable to the long eccentricity cycle 58, with an astronomically-tuned age of 23.03 Ma. The most important bio-events occurring close to the boundary are the first occurrence of planktonic foraminifer Paragloborotalia kugleri at 22.96 Ma and the last occurrence of nannofossil Sphenolithus delphix at 23.11 Ma. At ODP Site 1148 from near the Dongsha Islands, the Oligocene/Miocene boundary is characterized by a slumped deposit with several sedimentation breaks, indicating a series of wide scale tectonic activities in the then South China Sea.