Several experiments in which both p (spherical stress) and b (the coefficient of intermediate principal stress) remained constant on theπ-plane were performed on coarse-grained soil by the TSW-40 type true triaxial apparatus at Hohai University. The relationship between the peak value of stress ratio Mb and the value of b in these tests was studied. The test results indicate that Mb decreases with the increase of b, and the influence of b on Mb is greater when b is small, while the influence is small when b is large. The test results also show that Mb decreases with the increase of p under the same value of b, which shows that the strength of coarse-grained soil has nonlinear properties. The failure lines on the π-plane based on five failure criteria, including the failure criterion on a σ3- constant plane, the Mohr-Coulomb criterion, the stress-invariant failure criterion, the Lade-Duncan criterion, and the Matsuoka-Nakai criterion, were drawn and compared with the test results in order to verify the applicability of these criteria to coarse-grained soil. It is shown that the results of the failure criterion on a σ3-constant plane of coarse-grained soil are closer to the test results than those of the Mohr-Coulomb criterion, owing to consideration of the influence of intermediate principal stress, and it is convenient to take strength nonlinearity into account. It is also shown that the results of the stress-invariant failure criterion of coarse-grained soil are between those of the Lade-Duncan criterion and the Matsuoka-Nakai criterion, and they fit the test results better.