Abstract: Instead of the reducing tendency of the brittle fracture toughness with the increment of the thickness of the specimens, the elastic-plastic fracture toughness of the ductile material increases with the thickness of the specimens. In this paper, the effect of out-of-plane constraint to the fracture toughness of specimens is studied, which is defined as the different thickness of the specimens w varying with crack length a, while keeping the a/w as a constant. Experiments of ductile material fracture tests are carried out with the results: the elastic-plastic fracture toughness, J integral increases linearly with the thickness of the specimens. Theoretically, the energy release system is discussed according to the energy balance equations. With the energy release system, final results show that the fracture toughness J integral increases linearly with the thickness of the specimens, which are quite consistent with the experimental results.