Soils are generally penetrated by a pore system. Most transport processes usually occur in pore space of soils. The description of pore space shape is necessary to quantify how the geometry in-uences the percolation of substances such as water and nutrients. In this paper, the pores in soils are described by fractal geometry, and the fractal dimension is obtained from porosimetric measurements. The water permeability function is studied to correlate with the pore-size distribution of unsaturated soils, and is derived from the fractal model of pore-size distribution. The comparisons between the collected data of permeability and the proposed permeability function show that the calculations of the proposed permeability function are in satisfactory agreements with the measurements.