Abstract: In this paper, according to the concept of two state boundary surfaces and using typical normaliztd behaviour of normally compressed clay, a nonlinear model of soil stress-strain relationship is developed. The model can reflect both nonlinear behaviour of stress-strain relationship of soil and cross effect of spherical stress and deviatoric stress on the strain.Four moduli describing properties of deformation of soil are introduced.It seems reasonable to describ soil stress-strain relationship by four moduli.