Abstract: Flyashes from three power plants of China were studied with Free Vibration Test System. It is found that the dynamic shear moduli increase as confining pressure increases and decrease as viod ratio increases. The size grade and appearance of grain will also influence the shear moduli. The change of the degree of saturation will cause the change of dynamic shear moduli. By comparision, it is shown that dynamic shear moduli of flyashes are about 30~60% moduli of sand at same relative density.According to the comparison and analysis, we can infer that flyashes will most likely be effective isolation materials in ground.