Abstract An equivalent energy procedure has been proposed which uses a linear elastic fracture mechanics type formulation for predicting maximum load to fracture at any temperature. This formulation requires a knowledge of the fracture toughness parameter K I cd and the corresponding volumetric energy ratio. In this paper K I cd values as obtained from compact tension specimens are presented for ASTM A 533 grade B, class 1 steel at temperatures up to 550°F which cover the range of light water nuclear reactor vessel operating temperatures. Studies of impact energy ratios for the steel have indicated that K I cd values as determined from 1 T compact tension specimens may be almost numerically equal to the K I c values as determined from much larger specimens. This is shown to be true at temperatures up to which K I c values are known (50°F). These data indicate a very high resistance to fracture at higher temperatures even though a degradation is noted at 550°F.