Abstract: It has been reported many times that hydraulic conductivity decays with depth in aquifers throughout the world.In this paper, general models characterizing the decay of hydraulic conductivity with depth are given for porous media and fractured media, respectively. It is found out that the general models can be simplified by the exponential decay model under certain conditions and the decay exponent is convenient for describing the decay rate of hydraulic conductivity. Using this simplified model, the regularity of decay in hydraulic conductivity with depth in several aquifers with different depths of hydraulic conductivity measurements and lithology from different places around the world is analyzed. For aquifers with different depths of hydraulic conductivity measurements, the decay exponent decreases with the measurement depth. The decay exponent of aquifers with measurement depth of several meters is on the order of magnitude of 10-1 m-1, while that of aquifers with measurement depth of thousands meters is on the order of magnitude of 10-3 m-1. For aquifers with similar measurement depths, lithogy also has influence on the decay exponent of hydraulic conductivity. Therefore, the regularity of decay in hydraulic conductivity with depth should not be neglected in analyzing groundwater problems.