Abstract: The dynamic modulus tests of cyclical tri-axial loading with increasing and decreasing amplitudes were carried out for silty soils from one site in Taiyuan city and five sites in Handan city. The test results show that the speed of decrease in the dynamic modulus with the increase of the shear strain for the silty soils from different sites was slower, compared with sandy soil. The plastic threshold is found to be equal to 85% of Emax. So the amplitudes of cyclical loading were reduced just before the dynamic modulus gets to 85% Emax. As a result, the dynamic modulus would mostly return to the initial value. Variations of the clay contents have different functions. The dynamic strength of silty soils at their original conditions decreases with the increase of clay contents when the clay content is less than 10%. The dynamic strength of silty soil at their original conditions increases with the increase of clay contents when the clay content is more than 10%.