Abstract: The widely-graded gravelly soil has the potential to serve as an excellent anti-seepage material. However, the content of fine particles, properties of fines, compactive effort and grain-size distribution of coarse materials have great influence on the hydraulic conductivity of the widely-graded gravelly soils. Laboratory experiments are conducted to study the effect of above factors. The hydraulic conductivities of soils with different fines, coarse grains and mix ratios are measured using the variable water head permeability tests. The test results demonstrate that the permeability coefficient of the widely-graded gravelly soils decreases rapidly when the content of fines increases from 0 to 40%, and tends to be stable under higher value. The permeability coefficient decreases with the increase of liquid limit and plasticity index of fines. The permeability coefficient of silt and clay decreases exponentially with the increase of compactive effort. The continuity of the gradation of coarse materials can help to reduce the permeability of soils. According to the requirements of permeability of the core of earth dams, i.e., less than 1×10^-5 cm/s, the necessary mix ratio and properties of fines are proposed when the widely-graded gravelly soil is used as an anti-seepage material.