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Unsaturated permeability function can be estimated by theoretical models from soil–water characteristic curve (SWCC). To date, there are numerous estimation models that can be used to obtain the unsaturated permeability function from SWCC. However, each model results in a different estimation curve. The reason for this difference is not well understood. In this study, the available SWCC equations and the available relative permeability ( k r ) equations were combined to form a matrix of unsaturated permeability estimation models. The matrix of unsaturated permeability estimation models was used to study the effect of SWCC equations and relative permeability ( k r ) equations as controlling factors in the estimation of unsaturated permeability function. The study was conducted using twenty sets of published experimentally measured SWCC and unsaturated permeability data ( k w ). The effects of the SWCC equations and k r equations on a variation of estimated unsaturated permeability functions are presented in this paper. It was found that the SWCC equation has a more significant effect on the estimation of unsaturated permeability function than that of the k r equation. [ABSTRACT FROM AUTHOR]