Abstract: The critical state is one of the most important concepts in modern soil mechanics. But so far the clear definition of the critical state for unsaturated soils and the necessary conditions and constraints to reach the critical state for unsaturated soils have not been well given. Based on the theory of thermodynamics, the conditions and constraints to attain to the steady state of thermodynamic process for unsaturated soils are proposed, and it is pointed out that the steady state of deformation process for unsaturated soils is the ultimate of deformation process and it is just the concept of the critical state for unsaturated soils. The proposed conditions and constraints to attain to the critical state of deformation process for unsaturated soils are more general and complete with more rigorous thermodynamic theory, and are not just dependent on the results from the tests on the specific samples of the particular unsaturated soils.