Abstract: The Beishicheng and Hefangkou fault zones in Miyun and Huairou area have been studied.A mylonite belt tens to hundreds meters in width was developed on the foot wall of the former fault and a fracture belt 300—500 meters in width on the hanging wall.A mylonitized granite belt occurred on the foot wall of the latter fault and cataclasites or cataclastic granite on the hanging wall.Microscopically quartz and mica ,within mylonites exhibit their features of strong plastic deformation indicated by apparent flow structure and grain size reduction due to dynamic recrystallization.All minerals in the cataclastic rocks,however,display brittle behaviour.It is clear that the two kinds of rocks were formed under different physical conditions.Based on the distribution pattern of the fault rocks and dynamic analysis of twin lamellae of dolomite,it can be inferred that there were two phases during the developement of the fault zones: a ductile shearing occurred at the deeper level of the crust leading to the formation of mylonite belts,and then the mylonite belts gradually were uplifted along the south-eastern sides of the fault zones onto the higher level.According to the mineralogical assemblage and the characteristic of the mylonites,it is deduced that the ductile shearing occurred at temperature of 450—500°,and confining pressure of about 4 kb,corresponding to the depth of 15 km.A differential stress value of 0.5—1 kb and a strain rate of 1.6×10^-9—10^-10 sec^-1 have been estimated from rescrytallized grain size,subgrain size,dislocation density and the spacing of deformation lamellae of quartz revealed in chemically etched mylonite samples.Finally,the possible relationship between ductile shear belt,the elastic-plastic transition zone within the crust and the layer feasible to shake in continent has been discussed.