Abstract: The basic properties of some expansive soils such as composition, physic-chemical properties and microstructure etc. are studied in this paper. Especially, the factors influencing swell and contraction properties of expansive soil are thoroughly investigated. Several characterizing factors for swell and contraction properties (such as air-dry moisture we and free swell volume per unit dry weight VPS) and a new method for classification are used for the first time in the study of expansive soil.