The plastic zone formed around a notch tip is important in analyzing the fracture toughness of structures and particularly weld cracks existed in the weld HAZ (heat affected zone) which produces local plastic deformation at the crack tip. Therefore, in order to analyze the fracture toughness in weld HAZ, it is necessary to investigate the new fracture toughness parameter $K_{c}$ $^{*}$ and critical plastic strain energy $W_{p}$ $^{c}$ according to the shape and size of the plastic zone. 1) If the temperature corresponding to $K_{c}$ $^{*}$=130kg-m $m^{-3}$ 2/ is determined, transition temperature $T_{tr}$ the magnitude of plastic zone size, and heat input change depending on the fracture toughness. The blunted amounts of the parent and weld HAZ show mild linear variation until .delta.=0.4mm and then increase very steeply there after. 2) The relation between the plastic strain energy( $W^{p}$ ) and transition temperature( $T_{*}$tr) in parent metal is more sensitive than that of weld HAZ. However, the plastic strain energy depends on the transition temperature, and thus the yield stress, .sigma.$_{ys}$ becomes an important parameter for plastic strain energy. 3) The critical plastic strain energy( $W_{p}$ $^{c}$ ) absorbed by the plastic zone at the notch tip indicated in case of parent metal: 60J/mm, in case of heat input(20KJ/cm): 75J/mm, in case of heat input(30KJ/cm); 50J/mmJ/mm