Abstract: In the northern part of South China Sea, including Tonkin Gulf, Hainan Island, Leizhou Peninsula and some basins of Guangdong Province, Oligocene deposits are composed of motley, mainly grey, greyish green and dark red clay and sandstone, containing very rich pollen and spores. Most of them are of the components which exsisted in Tertiary, particularly in Oligocene deposits of Northern America and Eurasia, such as Gothanipollis paichanensis Ke & Shi, Ulmus granopollenites Rouse, Symplocos scabropollinia Traverse, Engelhardtia spackmaniana Traverse, etc. At the same time some important Oligocene and Early Miocene pollen types of Borneo are found there, such as Crassoretitriletes vanraadshooveni Germeraad & al., Florschuetzia semilobata Germeraad & al., F. trilobata Germeraad & al., Magnastriatites howardi Germeraad & al., Jandufouria seamrogiformis Germeraad & al., Verrucatosporites usmansis (Hammen) Germeraad & al., Margocolporites vanwijhci Germeraad & al., Psilatricolporites operculatus Hammen & Wymstra, Monoporites annulatus Hammen and Zonocostites sp., etc. On the other hand in Oligocene palynoflora of Borneo there appear pollen types assigned to temperate plants or mountain plants of northern hemispherae (Pinus, Picea, Tsuga, Ephedra and Alnus), which are found in abundence in Oligocene palynoflara of the investigated area particularly in Late Oligocene. All of this indicats that the palynoflora of the Northern part of South China Sea was closely related with that of Borneo in Oligocene. This similarity of palynofloras in the two localities may be due to the geographycal proximity of Borneo to the main land of Asia. In paleoecological aspect, we have found there is plenty of pollen and spores of land and fresh water plants, while those reflecting marine and coastal enviroments (Rhizophora and Sonneratia) are rather scanty. This shows that in Oligocene period the investigated area was a land. Tonkin Gulf along with Hainan Island and Leizhou Peninsula formed an inland depression at that time. Abundant pollen of mountain plants and temperate plants indicats, that there weremountains in or around the area of interest.