Abstract: In order to study special engineering properties （high porosity, high strength and low compressibility） of red soils, the physico-mechanical properties, chemical composition, mineralogical composition and microstructure features of samples of red soils in China have systematically analysed with SEM, EDAX, XDA and by conventional physico-chemical test.It has been found that the strength of red soil come mainly from chemical （atomic and molcular） bonds residual or regenerated between the particles, from cementing force of crystalline ferric oxide and from influence of colloidal ferric hydroxide.Though free colloidal ferric hydroxide existed in the pore solution under the humid condition have the certain restraint on clay surface activity, they can not prevent all swell-shrink behavior of clay.