Abstract: Hysteresis is a well-known phenomenon in the soil-water characteristic curves.It plays an important role in many aspects of unsaturated soils.A thermodynamic model of hysteresis for soil-water characteristic curves was presented.Internal state variables were used in the model to simulate the effects related to the capillary hysteresis,and they made the model more reasonable than the empirical models or the domain models.By introducing the concept of bounding surface,a general theoretical framework for hysteresis was developed.A simple relationship between boundary curves and the scanning curves was proposed,the irreversible component of volumetric moisture content was introduced as one of the internal state variables.An improvement of this model was that the slope of the scanning curves would not become infinite when they started from the boundary surface.The simplified model had only one parameter which could be easily determined if the boundary curves and at least one data point on the scanning curves were given.Model predictions were compared with the experimental results,showing that the model agreed well with the existing data.