Abstract: According to the analysis of the geomorphology, gravity, magnetic data of the extinct mid-ocean ridge in southwest sub-basin of the South China Sea (SCS), a relict rifting center is found in the study area,of which the feature is similar to that of Atlantic mid-ocean ridge. Four transform faults are identified along the mid-ocean ridge in spatial location. Each of the transform faults shows distinct geophysical features compared with the others. The fourth one is specially discussed in details. It might be an important transition zone between the continental rifting and sea floor spreading, which implies a different crustal structure on both sides of the fault.