Abstract: It is an important method to study the microstructure of soils by using the scanning electron microscopy (SEM). The recent researches on the microscopic porosity of the soils are based on the two-dimensional methods. The research methods mainly use the calibrated porosity to get the observation of the microstructural image of the soils and the apparent porosity. However, the microstructure by this way cannot conform to the change of the soil pores. Therefore, it is essential to learn the microstructure of the soils by using the SEM images and establish the quantitative relationship between the two-dimensional scanning image and the three-dimensional pore structure. Based on the SEM images and the double-integral method, a method for determining the three-dimensional unilateral apparent porosity of the soils with two-dimensional SEM images is proposed, and the comparitive tests on different soils are also carried out. The results show that the proposed method can reflect the three-dimensional porosity of the soils correctly.