Abstract: The non-coaxiality of constitutive relationship has strongly influenced the predictions of the inception of strain localization for soils.It was shown that since the classical non-coaxial theory was built only on the basis of 2D coaxial stress space,here an 3D non-coaxial constitutive framework was proposed based on 3D coaxial stress space and employed for predicting the onset of strain localization in soils.Comparisons between numerical predictions and experimental data were also provided.The analytical results presented that a non-coaxial plastic flow theory needed to be incorporated in the classical constitutive model to describe the bifurcation,which would lead to a correct prediction of the state of bifurcation.It was also concluded that the influence of the third stress invariant on the noncoaxiality,and that bifurcation analysis based on 3D model would get more precise results than 2D model under plane strain conditions.