Abstract: In this paper the experimental results of artifical over-consolidated clay sampleswith three kinds of stress path-consolidated undrained, consolidated drained andconstant mean effective principle stress tests are presented. It was shown that according to their overconsolidated ratio the deformation properties of over-consolidated clay samples can be divided into "wet" and "dry" zones by critical state line. In the "wet" zone, the samples are in light over-consolidated state （Rp = l⁴） and their deformation properties are the same as those of the normally consolidated clay, i. e. the hardening parameter may be expressed in plastic volumtric strain; the direction of plastic strainincrement （Δν/Δετ）p is independent of stress path, and the plastic potentail functionequals to yield function. So, those conclusions concerning normally consolidated clay have been expanded. In the "dry" zone, the samples are in heavy over-consolidated state （Rp = 5¹6）, the hardening parameter expressed in plastic shear strain is more suitable than the yield function expressed in failure line. Finally the generalised formula of yield function and hardening rule are proposed.