Abstract: Accurate modeling of water and air flow in unsaturated soils requires the reasonable definition of water retention behavior and the permeability behavior of water and air in the pores. By means of the approach developed previously to estimate the relative water permeability, a new approach to predict the relative air permeability is proposed based on the water retention curve. The power value in the approach can be considered as a decreasing exponential function of the coefficient characterizing the pore-size distribution of the soil and derived from its water retention curve. The model is calibrated using the data from 22 samples and validated using the data from 5 samples ranging from sand to silty clay loam in the existing literatures. The proposed approach is superior to the available alternative approaches for describing the evolution of relative air permeability with effective air saturation.