Abstract: Cyclic rotation of principal stress direction with constant amplitude is the cyclic stress cha- racteristics in seabed deposit induced by travelling waves. Results obtained from tests simulating the cyclic stress charcteristics,emphasize upon building-up of cyclic pore water pressure of sand sam- ples is presented. Regression analysis of test data showed that cyclic pore water pressure can be ex-pressed as the function of number of times of cyclic loading,as well as the hunction of generalized shear strain. Using results thus obtained,possibility of failure of seabed deposit under cyclic loading induced by travelling waves can be evaluated,By comparison with the test results of conventional cyclic torsion-al shear tests, it was shown that overlook of the effect of cyclic totation of principal stress direction will considerabky over-estimate the stability of seabed deposit.