Abstract: Theoretical consideration and method on evaluatng paleostress value of the fault zone are described by using the microstructure in mineral.The tectonic stress of the Yishui-Tongtou and Tongwu-Gegou ruptures on the norther part of the western branch of Yishui-fault zone in the late Yanshanian (post K₁) was calculated according to free dislocation density in deformed quartz crystal and percentage of deformed twin in calcite.The result shows that the stresses for the two crystals range from 1000-1500 b and 700-1100b,respectively.The microstructures of the fault zone show that the stable dislocation creep may occur both in the process of dry ductile/brittle-ductile deformation.Considering the effective water in the fault zone,it is likely that the evaluation can yield a real value by using McGomick relation rather than Weather's.