The variations of microstructures associated with the motion transformation in the central southern part of the Red River fault were studied in this paper. The researches included careful observation of microstructures in eighteen mylonite, twelve breccia, ten cataclasite, and eight sandy conglomerate samples from the fault, of which some were dated by the fission track in apatite (FT) method. In addition, the fault slip data were used to infer tectonic stress tensors. The results show that the microstructure of mylonite is indicative of left-slip during the early activity of the Red River fault. The FT ages of the mylonite and undeformed sandy conglomerate exceed 20 Ma, implying that a thermal event occurred in the Red River fault at that time. The microstructures of cataclastite and breccia imply the existence of right-slip during the late activity period of the fault. The above features, the FT ages （9.9～12.7 Ma, 6.8～8.4 Ma, 2.0～4.6 Ma）, and the different stress fields suggest that the Red River fault has undergone three dislocations since the Miocene, of which the first was strike-slip, and the second and third were of normal faulting.