The principle of interaction between plastic volumetric and shear strains for rock and soil has been extended to the field of unsaturated soils. Two new interactions of suction-plastic volumetric strain and pore air pressure-plastic volumetric strain appear in the unsaturated state of a soil except the interaction between plastic volumetric and shear strains. It is very important to find that the suction possesses a dual property, which is the origin of generating its special functions. Thereby the effect of the suction on volumetric strain includes two opposite aspects. By means of this property of suction, the physical significance of effective stress parameter, effects of suction on volume change and preconsolidation pressure, and the mechanism of collapse upon wetting all can be explained. In addition, it is theoretically proved by application of this principle of interaction that the critical state line for unsaturated soils exists, and is unique and independent of the stress history.