Abstract: The phenomenon of volume contraction in unloading is very common in the triaxial tests of soils. The magnitude of volume contraction declines when the confining pressure increases. Different densities of sands result in different volume contraction in unloading. To the dense sand, the reversibility of part of dilatation is the main reason of the volume contraction. To the loose sand, the volume contraction can be classified into two kinds-one appears at the initial stage of tests as a part of the plastic contraction and the other appears with the increasing shearing stress as the reversible part of dilatation. To clay, the volume contraction is totally resulted by plastic contraction, which indicates that the structure of soils destroyed with the loading unloading process.