Abstract: This paper focuses on the advance in fundamental behavior of unsaturated soil and slope stability. Some important properties of unsaturated soil associated with slope stability are discussed with emphasis on the fundamental problem of unsaturated soils, i.e., air/water phases and their movement."Four-stage air/water phase" division is put forward for an unsaturated soil in which the moisture condition varies from dry to saturated. Detailed experimental studies demonstrated that an unsaturated soil would exhibit different behavior at different states of air/water phase. Then, for unsaturated soil the suction in an unsaturated soil and its relationship to water content (i.e., soil-water characteristic curve(SWCC)) are discussed. A simple equation for describing the shear strength of unsaturated soils was proposed on the basis of the four-state division of air/water phase. The recent advance in the constitutive relationships of an unsaturated soil is also introduced briefly.After that, under the engineering background of the South-to-North Water Transfer Canal, the major internal and external factors affecting the slope stability in unsaturated expansive soil are discussed with the utmost concern to suction.Special discussions are put on the recent developments in the quantitative analysis for the effect of cracks on rainfall infiltration and shear strength of expansive soils.Following that, a full-scale field study on rainfall infiltration into an unsaturated expansive soil slope and a comprehensive laboratory study on the behavior of the unsaturated expansive soil are presented. On the basis of the valuable data obtained from the field and laboratory studies, some new findings for the mechanism of rain-induced landslides in unsaturated expansive soils are presented, and the slope stability analysis method considering the influences of crack and rainfall infiltration is presented.