In order to study the structural integrity of the reactor pressure vessel under pressurized thermal shock, both the cleavage and the ductile thermal shock fracture experiments using initially corner-cracked nozzle specimens made of Type A508 class 3 pressure vessel steel were performed. In both experiments, unstable fractures were realized, although the test conditions were very conservative compared to those of real plants. Finally, the three-dimensional and time-dependent fracture parameter obtained with the finite element method was successfully employed to discuss the fracture phenomenon.