Abstract: The experimental tests on loose sands subjected to undrained monotonic shearing with the relative density of 30% were performed by using the soil static and dynamic universal triaxial and torsional shear apparatus newly developed in Dalian University of Technology.A series of tests were conducted for loose sand under complex stress conditions with different coefficients of the intermediate principal stress,orientation of the principal stress and the mean principal stress.Based on the experimental results,the effects of orientation of principal stresses on the friction angles at phase-transformation state and peak stress state,stress-strain relations as well as effective stress paths were respectively examined.It was found that the undrained shear behavior of loose sands was considerably dependent on the orientation of the principal stress.