Abstract: According to BSI 7448 Part I, this paper designed three types of CTOD specimens with different thickness B of E36 steel, and CTOD tests were carried out at various temperatures to determine their ductile-brittle transition temperature curve. The test results revealed that E36 steel shows typical ductile-brittle transition characteristic and its ductile-brittle transition curve can be fitted well by Boltzmann function. What’s more, with the increase of the test specimen thickness(TST), the ductile-brittle transition curve shifts to higher temperature. In order to investigate the TST effect on the fracture toughness, an elastic-plastic finite element method (FEM) is used to analyze the stress distributions ahead of crack tips. We found that the out-of-plane constraint parameter Tzcan demonstrate the TST effect on the ductile-brittle transition temperature.