Abstract: Based on conventional classification system for unsaturated soils,the double-open unsaturated soil was further divided into the overlapped double-open unsaturated soil and non-overlapped double-open unsaturated soil according to the contact state of meniscus of pore water with the surface of the soil particles.Furthermore,the tension suction,which was closely related with but rather different from the matric suction,was presented on the basis of thermodynamics.Theoretical analysis on quantitative relation between matric suction and tension suction in unsaturated soils with isolated pore water was made.The equivalent suctions respectively corresponding to matric suction and tension suction were determined and compared by using the concept of equivalent suction given by the authors.Through the comparative study,the importance and essence of the tension suction in unsaturated soil mechanics was clearly displayed and the rationality of the concept of equivalent suction was illustrated.