Abstract: The Yi-She fault system is located in Shandong province and lies on the middle section of the Tancheng-Lujiang fault zone. The ductile shear zone found on the crystalline basement of the middle Yi-Shu fault system provides evidence for its formation in Presinian period Ductile shear zones have a NE or NNE trending and show a pattern of sinistral movement that turned Nw trending gneissosity in the west of the fault into NNE one. This great ductile shear deformation zone is about 20 km. in width. The distribution of the fault system may be controlled by the major shearing zone.The east part of the zone consists of the ductile shear zones in the lack of mylonite and original gneiss remnants,which suggests that it may result from gneissosity under the shearing at a relatively high temperature. However,the west part contains the shear zonet with various kinds of mylonite. They cut through and made original gneissosity move in such a way as sinistral movement,which reflects the result of displacesment under the condition of lower greenschist facies metamorphism.In this paper,the texture,deformation rocks and strain measurements of the shear zones with the mylonite are discussed. The deformation behavour of minerals in the matrix and mortar of mylonite,the fabric pattern and the preferred orientation of c-axis of the plastically deformed and syntectonic recrystallized quarz grains,and their evolution with progressive increase of the shear strain are described. The relationship of grain size and variation in content of feldspar with increasing strain in felsic mylonite is analysed. It is inferred that mylonite was formed at the crustal depth of 10—15 km. and had undergone a long time shearing at a strain rate of about 10^-14/sec. and a stress of about 580—745 bars.Finally,the ideal model of the major fault zone and the synthetical classification of fault rocks are discussed.