Abstract: The neotectonic activity of a minor fault zone on the Southwest margin of Yangzi platform has been studied based on the analysis and comparison of the microstructural features of deformed rocks within and outside the fault zone.The fault developed in Permian Emei basalt,striking NE40—50°,dipping toward Southeast at an angle of 70—80°,with a total length of 250m and horizontal displacement of 1.5m.Within the fault developed a zone 3—30cm in width consisting of fault gouge and fractured lenticular rock fragments,which along with the country rocks were cut by the late calcite veins.The samples were collected from the veins within the fault and the veins within the country rocks of different distance from the fault.The optical microscopic observation has shown that almost all of the calcite posseses twin lamellae,while U-stage measurement has shown that the space index of twin lamellae varies with the increasing distance from the fault.The space index is highest in the fault (50/mm),and decreases rapidly with the increasing distance from the fault (10/mm in the sample 20m away from the fault).The differential stress during the deformation process has been estimated from the developement degree of twin lamellae by using Jamison and Spang's method.The Result is in good agreement with that estimated from dynamically recrystallized grain-size.It is shown that the stress within the fault(700 bars) is much higher than that outside the fault(300 bars).According to thermoluminenscence dating the calcite veins were formed in Pleisto-cence.It is proposed that after Pleistocene there would be more than one active period of the fault.The stress of the main period was up to 700 bars.The strain and stress of the rocks within the fault during the faulting process were significantly greater than those in the rocks outside the fault.