Abstract: True triaxial tests on specimens of gravel were performed,in which σ3 was invariable,and σ1,σ2 were loaded at the same time in a certain proportion b =(σ2-σ3)/(σ1-σ3).The influences of the intermediate principal stress on three principal strains and strength were studied separately.Mohr-Coulomb Criterion,Lade-Duncan Criterion,Matsuoka-Nakai Criterion and Fang Kai-ze Criterion were compared with the true triaxial test results of gravel.It was found that the relationships between friction angle and b reflected by Matsuoka-Nakai Criterion and Fang Kai-ze Criterion were different from those of the experimental results,and the laws reflected by Lade-Duncan Criterion were close to those of the experimental results,while it overrated the influence of b on the friction angle.Hence a criterion was proposed,and the relationship between the friction angle and b was deduced and verified using several true triaxial test results.It was indicated that the proposed criterion could reflect the relationship between the friction angle and b more accurately than Lade-Duncan Criterion and Matsuoka-Nakai Criterion.