Abstract: Plane strain compression tests in which σ₃ keeps invariable and experiments in which both σ₃ and bb=(σ₂ -σ₃)/(σ₁ -σ₃) keep constant are performed on coarse-grained soil by means of the true triaxial apparatus recently developed in Hohai University. The characteristics of stress paths, parameter b at failure, angles of internal friction and peak values of stress ratio M _fp =( q/ p) _fp of coarse-grained soil under plane strain conditions are studied. The experimental results show that the stress paths of the plane strain tests exist between the b-constant tests in which b equals 0 and 0.25; the parameter b at failure is between 0.15 and 0.2 for coarse-grained soil under plane strain conditions; the angles of internal friction of coarse-grained soil under plane strain conditions are 5%~7% larger than those under triaxial compression conditions while the peak values of stress ratio of coarse-grained soil under plane strain conditions are 13%~18% smaller than those under triaxial compression conditions. Finally a formula expressed as M _fp =2.3sin φ _tc , which is verified by the test results of coarse-grained soil, is proposed to calculate the peak value of stress ratio, M _fp , under plane strain conditions from the angle of internal friction, φ _tc , obtained from conventional triaxial compression tests.