Abstract: The results of laboratory tests showed that the apparent preconsolidation pressure of Shanghai clay is higher than the effective overburden pressure. It's effects on the behaviour of soils are discussed based on the principle of critical state in this paper. The normal consolidation line, the critical state line and the strength envelope line are determined from the laboratory tests, and equations of these lines are given. In accordance with the variations of consolidation, shear strength, stress-strain relationships and pore pressure, it is concluded that Shanghai clay behaviours indicated as being slightly overconsolidated. The pc-effect should be considered in settlement calculatoins and stability analysis to improve the design methods.