Abstract: Liquefaction of gravelly soils is an unusual phenomenon during earthquake. Not more than 10 case histories are reported before “2008 Wenchuan earthquake”. The occurrence of gravelly soils has to meet strict conditions, including property of soils, intensity of earthquake shaking and burial conditions. Based on widespread liquefaction of gravelly soils during “2008 Wenchuan earthquake”, the hydraulic and geological background, permeability and drainage conditions of liquefied sites of gravelly soils are analyzed, and a typical case is selected to trench, and gravelly samples at liquefied layer are collected for shaking table tests and triaxial tests on specimens with diameter of 150 mm. The results reveal: (1) If the effect of membrane compliance is eliminated or neglected, the liquefaction resistance of gravelly soils is about identical to that of sand. (2) Using the pore water pressure model of Seed (1976), the normalized accumulative pore water pressure bulges up with the increasing level of cyclic stress. (3) For the reported case histories and liquefied situations during “2008 Wenchuan earthquake”, the drainage pass is almost clogged or the permeability of liquefied layer is low. (4) The increasing pore water pressure of loose gravelly soils and the decreasing effective stress under load of cyclic stress have two basic requirements. One is that the level of cyclic stress should be high enough to crush the soil particles or to destruct the structure of soil particles so that the gravelly soils become denser. As shear strain is up to the threshold strain of 0.02%, the pore water pressure begins to be generated, until the shear strain is greater than 0.1%, and the pore water pressure develops quickly even up to the total pressure. The other is that the drainage pass should be clogged or the permeability is low.