Abstract: The three-parameter Weibull distribution is used to characterize the statistic law of fracture toughness. The procedure to establish ductile-brittle transition curve of standard specimens is discussed according to ASTM E1921-05. In order that the experiment results can be applied to low constraint specimens, the two-parameter Weibull distribution is used to describe the relationship between local stress state in crack tip and failure probability. Then, crack tip stress field under different constraint levels can be matched each other by Weibull stress and the equation to predict ductile-brittle transition curve of ferritic steel is concluded. The finite element analysis is aim at two three-point bend specimens made of A533B steel and the relevant Weibull stress is performed. The predictions of ductile-brittle transition curve according to the procedure are well consistent with experiment data. This illustrates the validity of the present approach.