;Yichu fault which is a NE-SW trending, south dipping normal fault has been documented as a main structure offshore Tainan, southwestern Taiwan. This fault and faults nearby may be active suggested by seismic activity. High resolution seismic profiles currently indicate that densely active faults spreading about 20 km on the hanging wall of Yichu fault. Most of these faults are E-W trending with extensional displacement, penetrating the upper Pliocene strata at least. A1 fault is the most active antithetic fault, bounding the fault zoon in substance. On the other hand, sub-bottom profile provides fault scarp and growth-strata image upon the seafloor of Yichu fault and A1fault, respectively. Focal mechanism shows strike-slip stress transition field between the N-S extensional stress in Taiwan strait to the west and the oblique compressional stress on Taiwan orogeny to the East. By the observation, faults cease below Pleistocene submarine canyon system in the Eastern Penghu channel, Taiwan strait. By contrast, in the Western Penghu channel, those fault shows active reaching seafloor almost. The difference of fault geometry may be the result of stress field transiting from extension to strike-slip region.