Based on thermodynamics and stress theory of continuum mechanics

a series of stress state variables of unsaturated soils

which covered all stress state variables used in literature

are deduced from the equilibrium equation of soil skeleton. It is shown that there are more than five combinations of stress state variables for unsaturated soils

most of which are related to physical properties such as void ratio and degree of saturation. Stress state variables should have dimensions of stress

while void ratio

degree of saturation and their combination could not be regarded as stress state variables. Both the simplified Bishop’s effective stress equation and the Fredlund’s two stress state variables are simplified forms of stress state variables. The influence of void ratio is not included in the simplified Bishop’s effective stress equation

while the dependence on void ratio and degree of saturation are neglected in the Fredlund’s two stress state variables. When choosing stress state variables for unsaturated soils

rationality in theory and convenience in application as well as correctness in logic should be taken into account