The paper presents a simple critical state bounding surface constitutive model for describing the mechanical behaviour of over consolidated clays. Keeping the simplicity and the same set of the parameters as Modified Cam Clay model, new model provides a more realistic description of numerous elements of clay behaviour. The novel form of the hardening rule was proposed with the state parameter and the degree of overconsolidation as the state variables. Expressing the hardening parameter through the state parameter of the stress point on a loading surface and the state parameter of a conjugate stress point on the bounding surface, strain hardening and strain softening in drained conditions, as well as negative pore pressure in undrained conditions are well described. Inner loading surface always passes through the current stress point, thus enabling elastoplastic soil behaviour even in early stages of loading. The model overcomes many deficiencies of the Modified Cam Clay model as demonstrated on a broad experimental evidence.