The influence of dynamic strain aging (DSA) on deformation and fracture was investigated on ASME SA508-class 3 pressure Vessel steel. Serrated flow in stress-strain curves was observed between about 140 and 340 degrees C, which was varied by the strain rate and microstructural condition. The onset temperature of serration for the pearlite-ferrite microstructural condition was lower than that for the as-received, tempered martensite condition. The serrated flow range for both the microstructural conditions shifted to higher temperatures with increased strain rates. On the other hand, J-R curves were obtained by the direct current potential drop (DCPD) technique at three different loading rates (0.08, 0.5 and 3.0 mm/min). The Ji value for the crack initiation fracture toughness and the crack growth resistance dJ/da by the DCPD method were decreased by about 30-40% at the reactor operating temperature region. These behaviours could be attributed to a DSA effect. When the loading rate was fast, the minimum fracture toughness region by DSA was shifted to higher temperatures.