The deformation patterns and processes of granitic rocks determine the rheological behavior in the continental crust. In this contribution, we focused on the granitic ultramylonite in the South Tibetan Detachment System to investigate the microstructures, fabrics and compositions by various methods, including scanning electron microscopy, cathode luminescence, automatic mineral analyzer and electron backscattered diffraction. The mainly mineral compositions of the ultramylonite are Kfeldspar, plagioclase, quartz and biotite, etc. The microstructure is characterized by significant mylonitic foliation defined by compositional banding, namely, a combination of monophase domains and polyphase domains. The deformation temperatures estimated by phase equilibrium simulation and compositional isopleths of Ca component in plagioclase range from 390℃ to 410℃. The monophase domains are mainly divided into Kfeldspar domains and quartz domains. The microstructures of Kfeldspar domains show typical coreandmantle structures. Kfeldspars show intense fabrics and the similar CPO patterns for both porphyroclasts and matrix grains. The Schmid factor analysis for the porphyroclasts of Kfeldspar indicates that active slip systems are (100)\[010\], (010)\[001\] and (001)\[100\] during ductile deformation. The formation mechanism of Kfeldspar matrix grains is the dynamic recrystallization by subgrain rotation. Dislocation creep is the dominant deformation mechanism for Kfeldspar in the monophase domains. In the polyphase domains, the minerals are featured by finegrained matrix and only a few porphyroclasts exist. The matrix grains are mainly plagioclases and the micronsized biotites are widely distributed at the grain boundaries of the plagioclases. Plagioclases show random fabrics or weakly fabrics in the polyphase domains. The deformation mechanism is dominated by grain boundary sliding in plagioclase. In the monophase and polyphase domains, the finegrained quartz crystal show dissolution structures on their surfaces. The crystallized preferred orientation of the quartz caxes and the preferred orientation of the quartz morphology long axis are both parallel to the lineation direction. Dissolutionprecipitation creep is the major deformation mechanism in quartz. Fluid can make an important impact on the evolution from monophase domains to polyphase domains during ultramylonization. The fluid promotes the monophase Kfeldspar to break into finegrained plagioclase and quartz, which can further lead to the deformation mechanism transition from dislocation creep to nondislocation creep and the rheological weakening of the rock.