A new shear strength model is proposed for unsaturated soils to consider the capillary and adsorptive mechanisms. Firstly

two ideal states are defined for unsaturated soils

i.e.

the ideal capillarity and adsorption states

which are only affected by capillary or adsorptive mechanisms

respectively. Then the shear strength expressions of the two ideal states are proposed respectively. The product of the effective degree of saturation and the suction considering the cavitation of water under the tension is selected as variable for the ideal capillarity part

and the maximum apparent cohesion is selected as variable for the ideal adsorption part preliminarily. Secondly

based on the concept of the binary-medium model

the unsaturated soil is regarded as a media combined with the two ideal parts by a participation function. The weight of each ideal part represented by participation function is dependent of a cavitation probability variable. Then the new unsaturated soil shear strength with capillary and adsorptive mechanisms is established for unsaturated soils

and applicable to a wide range of suction. Finally

the comparisons between the published experimental data and the predicted results of the proposed and current popular models are carried out

which illustrates the superiority of the proposed model. Thus

the results have shown that the capillary and adsorptive mechanisms should be distinguished when considering the effect of suction on the mechanical property for unsaturated soils.