The plastic deformation during quenching is first induced by tension stress, and then alternative occurrence of tension and compression stress induce plastic deformation. The tension stress may induce compression stress. Compression stress occur in the surface, which is different from conventional view that only tension stress occurs in the surface. The thermal stress during quenching is mainly determined by temperature difference other than temperature. Compression stress occur and develop in the relative high temperature zone, and tension stress occur and develop in the relative low temperature zone. It is the source of thermal stresses, plastic deformation, residual stresses when quenching or quickly heating. The corner and edge models for thermal stress during quenching are established in this paper, and the corner effect, which go with the recession of, temperature grads, thermal stresses and plastic deformation after the first plastic deformation is induced by tension stress, is proposed in the course of quenching.