Abstract: The deformation behavior and strength characteristics of unsaturated expansive soils are studied by using modified triaxial apparatus.It is shown that defomation behavior and strength characteristics are affected by water content (suction)of unsaturated expansive soils.The plastic axial strain increases and strength decreases as water content increases.The relation between and suction is studied in this paper, and a new stress state variable for strength of unsaturated expansive soils is presented.