Abstract: Microstructure characteristics of representative expansive soils in China are studied.Micro-structure units in soils are summed up into three categories and microstructure characteristics,which consist of the structure units,are classified as six kinds.Relationship between microstructure characteristics and the expansive potential of the soils are discussed.The properties of smooth crack surface in the expansive soils,such as material component,microstructure orientation feature etc.are specially studied.Correlation between microstructure orientation degree and mechanical strength is obtained. Measurement results indicated that friction angle of the soils decreases with increases of orientation degree.