Finite element method was used to analyze the three-point bend experimental data of A533B-1 pressure vessel steel obtained by Sherry, Lidbury, and Beardsmore [1] from -160 to -45 °C within the ductile-brittle transition regime. As many researchers have shown, the failure stress (σ f ) of the material could be approximated as a constant. The characteristic length, or the critical distance (r c ) from the crack tip, at which σ f is reached, is shown to be temperature dependent based on the crack tip stress field calculated by the finite element method. With the J-A 2 two-parameter constraint theory in fracture mechanics, the fracture toughness (J C or K JC ) can be expressed as a function of the constraint level (A 2 ) and the critical distance r c . This relationship is used to predict the fracture toughness of A533B-1 in the ductile-brittle transition regime with a constant σ f and a set of temperature-dependent r c . It can be shown that the prediction agrees well with the test data for wide range of constraint levels from shallow cracks (a/W= 0.075) to deep cracks (a/W= 0.5), where a is the crack length and W is the specimen width.