The high-resolution X-ray micro-computed tomography (μ-CT) is used to investigate 3-D micro-structure of granitic residual soils. The results showed that: ① The water phase and their spatial distribution in soils can be determined by mixing the dried soil sample with CsCl solution. ② X-ray attenuation coefficient differentiation in different materials leads to easily distinguishing kaolinite and quartz in soils. ③ The spatial contact and orientation of soil particles are generalized and classified by using the reconstructed 3-D images. In addition, some further applications of the X-ray μ-CT to the study of unsaturated soil mechanics are predicted. These preliminary findings will contribute to the better understanding of the 3-D real structure of soil and to fostering new progresses of the X-ray μ-CT in geotechnical/geological engineering.