Abstract: In order to study soil microstructure quantitatively,three electrical resistivity indices, average formation factor, average shape factor and anisotropy factor, which can reflect the varieties of soil microstructure, are stated in this paper. Based on the electrical resistivity test during soil swelling, the evaluation system of the variety of soil microstructure is presented. Eventually, the feasibility of application of the electrical resistivity property indices on predicating the swell of expansive soils is discussed.