The top of geomembranes should be anchored firmly not to slip out. It is known that the thermal stress which occurs with the decrease in temperature should be a large portion of the forte which affects the anchoring section. Reliable equations for evaluating thermal stress cannot be found in literature. The authors conducted the chamber test under the oondition of confining with both ends of a sample and the large scale field test and evaluated the relationship between the temperature and thermal stress. As the result, it is found that (1) the thermal stress is much different with the type of a geomembrane, (2) the thermal stress of the test result under the condition of confining with both edges of a sample can be evaluated with the equation of the theory of elasticity, (3) when a vonfining force occurs by the weight of a soil layer, the thermal stress is smaller than the thermal stress under the condition of a strict confining. And the evaluating equation was presented.