We present a new two-parameter expression for relative hydraulic conductivity (RHC) of partially saturated soils. It is based on the premise of Assoulineet al. <1998> that soil structures evolve from a uniform random fragmentation process. This assumption allows us to derive hydraulic properties of soils (water retention curves and unsaturated hydraulic conductivity) from primary properties, such as pore geometry and soil structure. We tested our RHC expression against different soil types and found that it fits data better than the widely used models of Brooks and Corey <1964> and van Genuchten <1980>. ¿ 2001 American Geophysical Union