Abstract: Suction between grains of unsaturated soil is the foundation of establishing theory of unsaturated soil effective stress and shear strength. From the essence, concept, size of all suctions, several common suctions in research of unsaturated soil at present are compared and analyzed in detail this paper. Matrix suction represents the trend of absorbing water of soil, emphasizing the interaction between soil grains and water, not the interaction among soil grains. There are parameters x and φb in Bishop and Fredlund’s theory of strength, which do not have clear physical meaning, because their theories are in disagreement with the essential concept of effective stress. Though generalized suction involves the effect of structure suction, it lets matrix suction be a part of effective stress, making it practical, simple and more complicated. Additional inner pressure superposes the effect of matrix suction on surface tension suction, so reduplication of suction effect exists, and it doesn’t consider the effect of structure suction. Tension suction superposes tension suction on matrix suction, that is, two kind of suctions are component of surface tension suction along two grains connective direction. On the one hand, it still makes matrix suction a part of effective stress, on the other hand, it also doesn’t consider the effect of structure suction. Suction between grains (wet suction and structure suction) considers the effect of structure suction because of soil structure, at the same time, it also considers the effect of shrink film of gas-liquid interface, namely the effect of wet suction. Unsaturated soil effective stress theory and shear strength theory based on suction between grains are not only in agreement with the essential definition of effective stress, but also successfully explain so many features of unsaturated soil such as the directivity of shrink film tension, the average of concept of stress existing in soil, the uniqueness of soil physical essence, the continuity with the change in water content and the adaptability for different kinds of unsaturated soil. Therefore, studying effective stress theory of unsaturated soil from the suction between grains is correct and feasible.