Abstract: There is still much controversy over earthquake liquefaction of gravelly soils, and the mechanism explanation about earthquake liquefaction of saturated gravelly soils is similar to that of sand liquefaction. The cyclic dynamic triaxial experiments on some saturated gravel samples with 3 groups of typical gradations (gravel contents of 37%, 45% and 65%) and the diameter of 100 mm are carried out by employing the dynamic hollow cylinder apparatus. Based on the relationship curves of the shear stress-strain rate from experimental results, an obvious phenomenon is discovered that the saturated gravelly soils possess the similar curve characteristics with the saturated sands. The curve shape is altered from the elliptical shape under low pore water pressure state to the dumbbell one under high pore water pressure state. According to this phenomenon, the average flow coefficient and fluidity level describing the flowing property of the saturated gravelly soils are defined. The influences of the initial dynamic stress ratio, effective consolidation pressure, relative dense and gravel content on the relationship curves between the average flow coefficient and the pore water pressure ratio are discovered and discussed. The experimental results show that the initial dynamic stress ratio has little effects on the relationship between the average flow coefficient and the pore water pressure ratio. The flow level of saturated gravelly soils decreases as the relative density or gravel content increases. The influences of effective consolidation pressure on the relationship curves between the average flow coefficient and the pore water pressure ratio are relative to the gravel content. It is conjectured that the fluidity of saturated gravelly soils under cyclic loading should be determined by the contact state and the scale between gravels in the soils. It might be the fundamental distinction between the anti-liquefaction performance of saturated gravelly soils and sands that the contact state between gravels can not be released favorably under high pore water pressure. A new mechanism of the liquefaction of the gravelly soils is proposed and used to interpret those phenomena mentioned above.