Abstract: The existing methods were difficult to do quantitative researches on the microstructure of unsaturated clays.An electrical resistivity method was introduced into the microstructure studies on clayey soils and its effectiveness was evaluated.The swelling tests were performed on the compacted expansive soil samples with a modified oedometer consolidation cell,and at the same time the vertical and horizontal electrical resistivities were measured during swelling.The electrical parameters,such as the vertical structure factor,the horizontal structure factor,the average structure factor,the anisotropic electrical index and the average shape factor were then calculated and studied.It was shown that the electrical resistivity method was effective in quantitative studies on the variation of microstructure of expansive soils during swelling.