Abstract: A series of principal stress rotation tests are carried out on soft clay using a hollow cylinder apparatus equipped with high censers. The stress paths with fixed principal stress directions, pure principal stress rotation and cyclic principal stress rotation are studied. The influences of the intermediate principal stress parameter (b) and shear stress level (q) are taken into consideration. The effect of stress rotation on the small strain stiffness of soft clay is studied in particular. The test results indicate that the stiffness of soft clay is affected by the magnitude of strain under complex stress condition. The stiffness is relatively higher when the strain is smaller than 0.01%, and the stiffness decreases quickly during 0.01%~0.1%. The influence of shearing direction on the small strain stiffness is unobvious, while the influence of b is obvious. The influence of the pure principal stress rotation on the small strain stiffness of soft clay is dependent on the strain induced by the pure principal stress rotation. Strain is also accumulated during cyclic principal stress rotation, while the stiffness of soft clay is less influenced during fixed direction shearing, which indicates that the mode of the principal stress rotation also has significant effect on the small strain stiffness of soft clay.