Unsaturated soils are widely existed in nature.Marine soils will be in unsaturated state due to gas hydrates dissociation.Unsaturated soils show complex behaviours compared to saturated soils.A number of constitutive models for unsaturated soils have been proposed.However,naturally deposited soils are often overconsolidated.A few models take the influence of overconsolidation ratio into account.This paper presents a constitutive model,using Bishops effective stress and degree of saturation as independent state variables,in which the influence of the degree of saturation and the overconsolidation ratio can be properly described.The main advantage of the proposed model is that the hydro-mechanical behaviour of unsaturated soils is properly described.The validity of the proposed model was comfirmed by data from literature.