Abstract: Study on effective stress and on mechanic properties of unsaturated soils is based on the knowledge about the suctions between grains. In the light of the charateristic of the components of the suctions between grains in cohesive soil, the principle and method of testing the suctions are given and expounded, and the suctions between grains in red clay are measured and analysed.It is found that the influenecs of degree of saturation on the shear strength of cohesive soils are caused not only by the matrix potential in soil but also by the variable structure suction and absorbed suction. The large influences of the chemical composition of water in small opening on the suctions between grains in soils are different from those on the variable structure suction and absorbed suction.