To make a realistic prediction of ground deformation, the constitutive model and the input parameters should represent the field conditions. Laboratory tests generally are used to determine the input parameters for the selected constitutive models controlling various stress and drainage conditions, but laboratory tests have disadvantages in that the samples from the bore hole which are prone to be disturbed by the tube penetrations, sample preparations, storage and so on. To overcome these disadvantages, it is necessary to understand the effect of disturbance on the stiffness of the sample, especially the normally consolidated clays which generally are considered as soft clays. Therefore, in this study triaxial tests are performed on the normally consolidated kaolinite to evaluate the sample disturbance effects on the stiffness and determine the field representative input parameters.