Stress variable is one of the key issues in constitutive modelling for unsaturated soil. Unfortunately

this issue is often ignored and may impede the further development of constitutive modelling. This article investigates the stress variables from the viewpoint of soil microstructure

conservation of energy and mechanical equilibrium

on which the selection principle of stress variables of unsaturated soil is put forward based. The difference between stress variables and stress state variables is highly focused. The study results show that some variables

like matric suction and net normal stress

which have the units of stress

are insufficient to qualify them as stress variables on the scale of the air-water-solid REV

but they are the essential stress state variables

and the effective stress and the suction stress could be the stress variables of unsaturated soil. Finally

we prefer using effective stress to simulate constitutive unsaturated soil behaviour

not only because it can obey the above principles but also it can smoothly switch to the theory of saturated soil. It is illustrated that the principle of stress variables selection for unsaturated soil is still immature

but the attempt to develop this theory is requisite.