Engineering projects like embankment,riverbank and earth-rock dam in the loess area are mainly built above groundwater level.The loess is in unsaturated state and subject to plane strain.The unsaturated undisturbed loess under the plane strain state is studied by using reformed TS-526 true triaxial apparatus.Results show that deformation and strength of the unsaturated loess vary with water content obviously.The more water content,the less strength of the loess.The deviatoric stress-curves of the loess are hardening and overhardening as axial strain increases and however,there is an obvious inflection point on the deviatoric stress-volume change curves.The shear strength parameter of the loess decreases with increased water content.Water content has a major effect on deformation and strength of the projects.