Abstract: On the basis of the efficiency of the compacting effect of natural loads on clay soils, under-consolidated clay soils are divided into two categories, i. e. common under-consolidated clay soil and one with structural strength. The geological condition of the formation of under-consolidated clay soil is analysed. The inducing sources of forming bonds of hardening between soil particles are discussed. Owing to the bonds and high porosity in such soils, its characteries of compressibility, the change of pore-pressure and shear stress-strain relationship all depend on stress level.