Constitutive models that introduce the state of the soil as an important determinant of the mechanical behaviour have a great potential for practical application. Simple formulation, clear physical meaning of the parameters, reliable prediction of a wide range of soil behaviour are the benefits of the state-based constitutive models [1]. It was shown in the paper that the state parameter concept that combines the influence of normal stresses and void ratio, primarily developed for the coarse-grained soil, can be effectively used in the constitutive relations of the fine-grained soil. Hardening State Parameter model (HASP) for describing mechanical behaviour of overconsolidated clays is developed on the basis of the critical state theory and within the concept of bounding surface plasticity [2]. With the state parameter and the degree of overconsolidation as the state variables a novel form of the hardening rule was proposed and it is possible to describe a number of elements of the mechanical behaviour of overconsolidated clays. In undrained conditions, the general form of the effective stress paths depending on the overconsolidation ratio is well predicted. In drained conditions, HASP model predicts the smooth transition from contractive to dilatant behaviour, and a smooth transition from hardening to softening, without an explicit mathematical description. Possibilities of the HASP model to adequately predict the behaviour of overconsolidated clays have been tested through the analysis of published results of drained and undrained triaxial tests in compression and extension. Reliable numerical prediction of the soft overconsolidated clay behaviour due to embankment loading is achieved. Since the parameters of the HASP model are obtained from standard laboratory tests, it can be easily applied for routine geotechnical analyses.