In 2011, an oil spill occurred off LachHuyen port in the northeast of Vietnam, due to vessels collision. Meanwhile, winds prevailed over the South and Southwest at Bach Long Vi station during July and August; the oil spill would have moved quickly and spread out around Cat Ba Island. Strong waves also speeded up the spread of oil. In this case, MIKE21 SA model was applied to simulate the oil spill trajectory, and simulation results showed the direct impacts on the ecosystem of Cat Ba biosphere reserve. MapInfo 9.0 was applied to build up environmental sensitivity maps for Cat Ba biosphere ecosystem. Results of the spill simulation combined the MIKE 21 SA and Cat Ba ecosystem maps give us an overview of the impact to the Cat Ba biosphere ecosystem. In fact it can really be helpful to make prompt response for crisis and proceed rescue measures for oil spill incidents in the future.