Abstract: Based on thermodynamics theory and foundational principle of soil mechanics, geometrical relation between ball-shaped particles and pore water is employed to investigate the force system in unsaturated soils induced by unsaturation. And then definitions for tension suction and suction ratio are construted. Another concept named equivalent suction is put forward after magnitude of suction as well as correspondiog effective area of matric suction and tension suction on particle surface are taken into consideration. Study on an ideal model shows unsaturated soils can be rationally divided into overlapping unsaturated soils and non-overlapping unsaturated soils according to whether meniscus overlaps on particle surface or not. Tension suction only appears in overlapping unsaturated soils and contributes to strength but not to seepage is another achieved conclusion.