Abstract: In this paper,we will compare and analyse the geologic and geophysic characters of the Eastern and Southwestern Sub-basins of the South China Sea(SCS)and discuss the SCS’s tectonic evolution history. The results of studies suggest that the magnetic anomalies and the gravity anomalies of Eastern Sub-basin are in EW direction but in NE direction in the Southwestern Sub-basin.In both Sub-basins the Bouguer gravity anomalies in the central part are bigger than in the margins. This indicates that below the central part exists high density matter,which could be closely related to the spreading of the basin and the invasion of high density matter from the mantle. A computational process called the Interpolation Cut Methods was used to ana1yse the magnetic anomalies of the two Sub-basins.Results of the analyse indicate that the magnetic anomalies of the Southwestern Sub-basin is mainly affected by the shallow magnetic sources above about 5km in NE-SW direction. Below 5km ,the magnetic anomalies become blur,and the intensity decreases.The magnetic anomalies of the Eastern Sub-basin is not only affected by the shallow magnetic source above 5km ,but also affected by the deep source. They are all in EW direction.The differences between the two Sub-basins reflect the different tectonic mechanisms. Most faults in SCS exist in the conjunctional area of the Circum-Pacific tectonic belt and the Tethys tectonic belt and are effected by both belts.The faults in SCS basin is mostly in NE and near EW directions.The NE direction faults formed the structure frame work of the Southwestern Sub-basin and the western part of the Eastern Sub-basin.The formation of the NE direction faults are same in the mechanism and time as the faults in the Eastern China, which were commonly formed in Yanshanian stage. In the Himalayan stage,because the main tectonic point moved to east,the NE direction faults of the Southeast China formed earlier and was relative1y stable;the EW direction faults formed later and then cut across the NE direction faults. Because the EW direction faults cut very deep and can even reach the lithosphere,they often formed the gushing channels of earth mantle. The South China Sea is located in the conjunctional areas of the Pacific and the Tethys tectonic belts.Its evolution was affected by both the tectonic belts.The SCS was affected by the Pacific tectonics same as the Eastern China.Both SCS and the eastern China were in back—arc extension tectonics of NE-SW direction.These areas were also affected by the Tetheys tectonics.In this stage,the Tetheys oceanic crust finally subducted under the Eurasia plate.Combined with the strong NE-SW extensional stress,extensional stress in NW-SE direction was formed in the SCS.Therefore, in the action of the two stresses,many graben basins in NE direction appeared in the SCS. As the extension were stronger than in other places of the Eastern China,the SCS crust was completely taken up and intruded by deep magma.Basic-ultrabasic rocks in the crust are the shallow magnetic sources in the Southwestern Sub-basin. After Eocene(42-43Ma)we saw the most important change in the Pacific boundaries.Its subduction direction changed from NNW to NWW. The Pacific subducted at the Philippine trench beneath the Eurasia and formed NEE-EW direction rifts between the Zhongsha-Xisha islands and Nansha Islands.According to the reconstruction of the SE Asia,Hall suggested that the opening of the West Philippine sea and the West Sulawei Sea triggerd the Southward subduction of the Proto-S uth China Sea at the North Borneo-Luzon Arcs,and caused the N—S orientation extension of the South Chinese margin and the southward slab-pull force.Simultaneously the Indian began its collision with the Eurasia and led the uplift of the Tibet and c1ose of Tetheys.For the continuous collision ,the mantle moved in large scale on SE direction and the Indochina plateau extruded.The Red River zone started its left-lateral slip nearly more than 500 kms.In the control of the NEE-EW direction faults in the north margin and the NW direction olle in the west,the micro-plate composed of the Zhongsha Is,Xisha Is and the Nansha is broke away from the main continental and moved southward.Finally The sea floor spreading like the oceanic ridge took place.The Eastern sub-basin in EW direction came into being.The pattern of the magnetic anomalies and the structure are all in EW direction.