The unsaturated zone is a complex multiphase system, and modelling and prediction of flow and contaminant transport in this zone remain a challenge. In order to understand the mechanisms of fluid flow in unsaturated sands, an accurate and efficient approach to estimate unsaturated hydraulic conductivity (K) is essential. In this study, a power law relationship was derived from a combination of Archie's law and van Genuchten's model to relate bulk (apparent) electrical conductivity ([EC.sub.a]) with unsaturated K. The laboratory sandbox experiments were conducted first to delineate the soil water characteristic curves (SWCCs). Time domain reflectometry was used to simultaneously measure volumetric water content ([theta]) and [EC.sub.a]. Then, the experimental relationships of the effective saturation (S) and [EC.sub.a] and simulated S-K were combined to establish the relationship between [EC.sub.a] and unsaturated K. The developed power law relationships described the relative EC ([EC.sub.r]) and relative K ([K.sub.r]) very well by just using one parameter, exponent [beta]. When fluid EC was low, the [beta] values for the drainage and wetting processes ranged within 2.09-2.74 and 2.50-3.79 respectively. The variations of [beta] values of homogeneous material were smaller that of heterogeneous material and the effect of hysteresis on the [EC.sub.r]-ATr relationship was observed. When pore space was filled with the high-EC solution, it easily mimicked the S-[K.sub.r] relationship and resulted in a smaller [beta] value. The [beta] value acted as a lumped factor accounting for pore tortuosity, pore connectivity, shape of pore space, and fluid EC. The power law relationship of [EC.sub.r]-[K.sub.r[ developed in this study could lead to a direct estimation of the spatial and temporal variations of unsaturated K, once the measurements of SWCC are available from estimation of saturated K and combination of time-lapse [EC.sub.a] measurements. Accurate and efficient estimation of unsaturated K could improve the prediction of flow in the unsaturated zone and allow a comprehensive understanding of unsaturated zone processes. Keywords: electrical conductivity, heterogeneous, homogeneous, hydraulic conductivity, power law relationship, soil water characteristic curve, time domain reflectometry, unsaturated sands.