In order to determine the effect of grain size distribution on the unsaturated hydraulic properties of rockfill dam material, a series of large-scale laboratory tests have been conducted by using well graded samples with similar grain characteristics. The experimental results show that (1) the water retention characteristic curve of coarse granular material can be obtained precisely by using the Column method, (2) the unsaturated permeability coefficient can be obtained by establishing a steady state of the sample-mass, in maintaining a constant water flux on top of the soil column, and (3) the Van Genuchten model, which predicts very well the water retention curve of porous materials, do not apply to unsaturated permeability coefficient of such materials. Rather, the Irmay model predicts well the unsaturated permeability coefficient of such materials.