Abstract: Using the true triaxial apparatus recently developed in Hohai University, a series of unilateral loading tests are performed on coarse-grained soil, in which two of the three principal stresses are invariable, and only one increases. The experimental results show that the deformation in the loading direction is compression in all cases, while the deformations in the other two lateral directions are complicated. When loading is applied in the direction of the major or the intermediate principal stress, the deformation in the lateral direction of the minor principal stress is dilatancy and few deformation occurs in the other lateral direction which is the corresponding direction of the intermediate or major principal stress; when loading is applied in the direction of the minor principal stress, the dilatant deformation occurring in the direction of either the major or the intermediate principal stress is very little. It is indicated that the dilatant deformation is mainly caused by the increment of the shear effect. That is to say: if the shear effect increases, the dilatant deformation will occur; if the shear effect decreases, there will be little dilatant deformation. The instance with loading applied in the direction of the minor principal stress is studied. It is found that the variations of p, q, b all make the stress state away from failure surface, consequently the shear effect decreases and little dilatant deformation occurs. The tested soil exhibits strong anisotropy in general stress states.