A　constitutive　model　of　unsaturated　soils　is　developed　by　incorporating　the　concept　of　intergranular　stress　into　the　framework　of　the　modified　Cam-Clay　model.　Within　this　context,　the　degree　of　saturation　is　viewed　as　an　internal　state　variable,　so　that　the　soil-water　retention　function　appears　naturally　as　an　evolution　equation　for　the　volume　fraction　of　water.　The　new　model　not　only　has　a　neat　structure　but　also　includes　fewer　material　parameters　compared　to　the　other　models.　A　new　volumetric-hardening　law　is　proposed　by　taking　into　account　the　effect　of　wetting-induced　pore　collapse.　It　is　shown　that　the　collapsing　void　ratio　(the　current　void　ratio　minus　the　void　ratio　at　full　saturation　under　the　same　effective　stress)　is　dominated　by　the　degree　of　pore　air　saturation　(one　minus　the　degree　of　water　saturation)　and　practically　independent　of　intergranular　pulling　forces,　highlighting　the　important　effect　of　soil　fabric　on　the　wetting-induced　pore　collapse.　The　proposed　model　is　applied　to　simulate　different　types　of　experiments　on　unsaturated　soils　subjected　to　various　combined　hydraulic　and　mechanical　loadings,　showing　its　capability　and　diversity　in　modeling　the　hydraulic　and　mechanical　behavior　of　unsaturated　soils.