Abstract: The hydraulic conductivity of unsaturated soils controls their water flow. Most numerical softwares adopt the hydraulic conductivity function (HCF) as the input parameter to simulate the water flow in unsaturated soils. The accuracy of the inputted HCF has significant effect on the reliability of the simulated results from the seepage analysis. It is noted that most HCFs are proposed based on the theory of capillary flow. It is observed that in the high suction range, the estimated HCF of unsaturated soils from the capillary flow is much lower than what is obtained from the experimental measurement. In this study, the water flows in unsaturated soils are categorized into three forms, including (i) capillary flow, (ii) film flow and (iii) vapour flow. The mathematical models for the HCF of soils corresponding to those three forms of water flow are proposed. Consequently, the total HCF of unsaturated soils under the entire suction range is proposed.