Abstract: A thermo hydro mechanical constitutive model for unsaturated soils is proposed based on the existing CAP model for unsaturated soils. The influences of temperature on the hydro mechanical behaviour of unsaturated soils are described in the model. Particularly, the thermal softening phenomenon, i.e. decreases in value of the pre consolidation pressure and in critical value of the suction of the SI (Suction Increasing) curve with heating process, are quantitatively modelled by using the experimental data and previous work.Comparisons of the results obtained from the numerical modeling of the constitutive behaviour with existing experimental results show that the present model can simulate the thermo hydro mechanical behaviour of unsaturated soils satisfactorily.The versatility and applicability of the present constitutive model are proved by the numerical results given in this paper.