Use of a sophisticated model to conduct simulation analysis of both contractive and dilative behaviours in unsaturated soils during the shear stage remains challenging. This paper presents an elasto-plastic model having two plastic deformation mechanisms—compression and shear sliding—for unsaturated soils, using the average soil skeleton stress σ'ij as a stress variable and void ratio e and effective degree of saturation Sre as state variables. With a view to well describing dilative behaviour for unsaturated soils, unlike in previous studies, Sre-dependence of a critical state line in the e–log p' plane is explicitly implemented by means of a formula that accounts for the influence of pore pressures and additional “bonding” relating to water menisci under an unsaturated state. The new model's ability to reproduce the shear strength and dilative and contractive behaviour of unsaturated soil are evidenced through simulation of triaxial tests on silty sand, Speswhite kaolin and Jossigny silty clay.