Abstract: The soil water characteristic curve (SWCC) is an important hydraulic parameter for modeling water flow and contaminant transport in the unsaturated soil. However, direct measurement of the SWCC is still difficult. The Arya and Paris (AP) model estimates the SWCC from particle-size distribution curve (PSD) based on the shape similarity of the two curves. It introduces an empirical parameter, a, used to scale pore attributes from hypothetical formations to natural structures. Several approaches are used to derive a. However, the calculation procedures of these approaches are either quite complicated or developed without paying much attention to the physical significance of the soil properties. In the present paper the physically based scaling technique (PBS) is employed to derive a for the AP model. Fifty soil samples, representing a range of textures that include sand, sandy loam, loam, silt loam and clay, are selected from UNSODA hydraulic property database for calculating a using the PBS approach. In addition, nineteen soil samples with different textures are used to verify the effectiveness of proposed a values. The results are compared with those of other approaches and show that the PBS technique combining with the AP model is a more useful and easier approach to predict SWCC from PSD.