In this paper, the authors studied the variations of microstructures associated with the motion transformation in the central-southern section of the Red River Fault. The work includes careful observation of microstructures of the 18 mylonite, 12 breccia, 10 cataclasite, and 8 sandy conglomerate samples from the fault, some of which are dated by the fission track in apatite (FT) method. Meanwhile, fault slip data are used to infer tectonic stress tensors. The results show that the microstructure of mylonite is indicative of left-slip during the early stage of movement of the Red River Fault.