Abstract: The liquefaction of the natural gravelly soils is a new problem beyond the existing awareness and codes, and the trigger conditions of liquefaction are critical. It is the most reliable to extract the relevant knowledge from field investigation of earthquake damages. In view of the world's largest scale of gravelly soil liquefaction in the 2008 Wenchuan earthquake, using the post-earthquake field investigation data from the meizoseismal area in the Wenchuan earthquake and the historical documents on liquefaction of gravelly soils in the world, the necessary trigger conditions for liquefaction of gravelly soils and the relevant characteristic parameters are studied. The analytical results indicate: (1) The minimum ground shaking of 0.15g is essential to trigger liquefaction of natural gravelly soils, yet the liquefaction of massive gravelly soils requires 0.20g to 0.40g of ground shaking intensity; (2) The most natural gravelly soil layers are very loose. Even though the gravelly soils can be compacted with the increasing ground shaking, the compacted gravelly soils are still slightly dense. (3) The gravel contents of the liquefied soils can reach 85% and even larger in reality, and they do not decrease with the decreasing seismic intensity; (4) The layers of gravelly soils with high shear wave velocity can be liquefied. The shear wave velocities dividing different compactions for sands and gravels are different, and the liquefaction evaluation methods for sand cannot be applied in gravelly soils. (5) The existence of a cap of overburden low- permeability and a non-liquefiable layer with minimum 0.5 m in thickness is a necessary condition for possibility of liquefaction of gravelly soils, which can be defined as the cap effect. (6) The gap for drainage between the underground water table and the overburden cohesive layer cannot be large, which is another necessary condition for possible occurrence of liquefaction of gravelly soils, and the thickness should be less than 2.0 m, which can be defined as the gap effect. (7) Different from liquefaction evaluation of sandy soils, the buried conditions of layers of gravelly soils must be considered in liquefaction evaluation, otherwise the liquefaction possibility of sites is easily misjudged.