Abstract: For numerous constitutive models are established based on the coaxial theory and validated only under triaxial compression condition,the soil behavior is inaccurately described under the true triaxial condition.A 3D non-coaxial elasto-plasticity model with a novel su?rface shape was proposed to simulate stress-strain curves before bifurcation and to predict the bifurcation strength.It was shown that both the pre-bifurcation deformation and the bifurcation strength were strongly affected by the intermediate principal stress,and the peak failure stemmed from the onset of strain localization when the ratio of intermediate principal stress was beyond 0.2.It was shown by comparisons between the predicted and experimental results on dense sands that the 3D non-coaxial model was more reasonable to be used to predict the bifurcation strength than the coaxial one under the true triaxial condition.