Ductile fracture tests with two distinctly different loading rates, 0.08 mm/min and 3.0 mm/min, have been performed on the tempered and quenches SA 508 class 3 nuclear pressure vessel steel (PVS) in the range of dynamic strain aging (DSA) between room temperature-350℃. Fracture property data were generated using single specimen J-test where crack length were measured by direct current electric potential technique. Ductile fracture results as a function of loading rate indicate that the faster the loading rate, the higher DSA temperature under static loading condition. This characteristics is attributed to interaction between interstitial carbon atoms and mobile dislocations in the plastic zone formed at crack front. As a result, there was an attempt to explain the toughness variation with loading rate by means of applying negative strain rate sensitivity concept to fracture process. By comparing crack growth resistance (dJ/da) with crack initiation toughness ($J_i$) as a fracture criteria, it was confirmed that tearing modulus was more accurate parameter in order to judge instability of ductile and tough material such as nuclear pressure vessel steel. Finally, it was concluded that d-c EP was more sensitive crack monitoring technique by means of comparing this test results with existing data obtained using unloading compliance method.