Abstract: The highly precise measurements of deformation of soils in the field and laboratory have shown that the response of these soils is non-linear under very small strains.The soil behaviors can not be realistically described by the normal laboratory tests and traditional linear elastic analysis,and then it is vital to use the in-situ tests and non-linear analysis to do so.Based on the theory of non-linear elastic/perfectly plastic analysis for plane strain undrained expansion tests proposed by Bolton et al.,experimental investigation on the decay of stiffness with strain during the Cambridge self-boring pressuremeter tests is carried out,and a comparison between the non-linear analysis and the linear elastic analysis is drawn.The results of test and analysis indicate that the stiffness of soils exhibits non-linear behaviors under very small strain and falls with the increasing strains,but the amplitude of decrease varies in different ranges of strain.The secant modulus of soil is usually larger than the tangent one,but the value of differences decreases with the increasing strain.It is apparent that the simple linear elastic solution greatly underestimates the range of the elastic response and the strength of soil.On the contrary,the value of stiffness corresponding to various strains can be deduced by non-linear analyses,which reflects realistically the non-linear behaviors of the soil under small strain.