Abstract: The framework for the behaviour of unsaturated expansive clays (G-A model) proposed by Gens and Alsono can explain the mechanical behaviour of expansive clays, in which the fundamental characteristic is the explicit consideration of microstructural and macrostructural deformations. The constitutive models based on the framework of G-A model have a lot of parameters which are hard to be determined. The constitutive model in this work deduces the neutral loading lines in the framework of SFG model. The total deformations reflect micro-level deformation, therefore it is not necessary to distinguish between micro-level and macro-level structural deformations. It allows the constitutive model for unsaturated expansive clays to reduce parameters. A mathematical formulation of the model is assessed by comparison with the experimental data available in literatures. The model predictions agree with the experimental results.