Abstract: Volume contraction in unloading is known in shear tests of most kinds of soils. It is shown that the behavior of soil is related to physical properties of soil, stress path of test, boundary condition in test and it is also related to anisotropy of soil. The volume contraction in unloading of soil results from the reversible dilatation of soil. In the process of the dilatation, the soil particles are transferred from lower potential energy state to higher one and the latter is an unstable state. Therefore, the most of volume strain due to dilatation will revert when the shear stress is unloaded.