Abstract: The formation of shear band and the theory of progressive failure of soils are key topics focused greatly both in solid mechanics and geomechanics around the world. In this paper, based on the results of triaxial compression tests on structured samples, macro-mechanical conditions for the formation of shear band and its inclination angle are discussed at first.Then microstructures of the soil inside and outside the shear band are studied by the means of Scanning Electron Microscope and quantitative techniques.It has been confirmed that sufficient structural strength is one of the conditions for the formation of shear band for structured clay.Inclination angle of the shear band is not in accordance with Roscoe′s theory. Thickness of the shear band is different on macro scale and on micro scale. For the soils inside and outside the shear band, a great difference is shown in pore distribution, porosity, orientability, anisotropy, etc.. Both dilatation and contraction exist on the shear plane, while the latter exhibits on the whole. Inside the shear band a great contraction is captured. But the whole sample shows slight dilatation macroscopically. All these results not only verify the Masonry theory proposed by Shen Zhujiang, but also make good bases to determine the indexes of soil structure, and to develop the structural models and progressive failure theory for soils.