Abstract: The principal stress of soil in the direction normal to plane strain is usually considered to be the intermediate principal stress,however it is true only when the sample is nearly failed in plane strain condition. In present paper, based on test results and theory analysis, some interacting conclusions are drawn. In the plane strain loading tests where the ratio of major active principal stress to minor one keeps a small constant, the principal stress of soil in the direction normal to plane strain is generally the minor principal stress. In the plane strain tests where active principal stresses decrease to small values, the principal stress of soil in the direction normal to plane strain may become the major principal stress, that indicates the yielding of soil under decreasing load and results from elasto plasticity of soils.