There have been two periods of volcanism with 5 eruptions in the area of northwesterta Hainan Island. The first one occurred in late Pleistocene, dating back to 5.2 X 10⁴ a B. P..It formed a lava field which covered a sandy deposit. This was later baked and altered. Ano-ther eruption took place 2.5 X 10⁴ a B. P. The lava which emitted at that time consists of olivine basalt and tholeiite basalt; the flow ripples of elastics of them are still well-preserved. Later eruptions occurred in the Holocene, which many small breached craters appearing on the coasts and islands around and in the Beibu Bay. The last of these eruptions raised the former shingle beaches 15 m high and the former tidal flat to form terrace 20 m above sea level. This volcanic belt runs along the seashore which trends in northeast direction. It might be the result of a crustal extension in the Beibu Bay and Yingge Sea, Since the earth crust here is only 30 km thick. At present barrier coral reefs are well-developed along this volcanic coast. Most probably, they have basalt thus emitted as their basement, since the geothermal gradient here is favorable for their growth.