The effects of dynamic strain aging (DSA) on the fracture resistance of ASME SA516-Gr.70 steels for nuclear piping were investigated through J-R tests conducted in the wide ranges of temperatures (RT-500℃) and loading rates (0.3-1000 ㎜/min). Tensile properties were also investigated in the temperature range of 100 to 316℃ over a range of strain rates (2.5×10^(-5)-1.33×10^0/s). The test results indicated that the SA516-Gr.70 steel was very susceptible to DSA. The minimum fracture resistance values were obtained in the temperature range between 200℃ and 400℃ for each given loading rate. The temperature for the minimum moved to higher temperature region as loading rates increase. The minimum was discussed in the viewpoint of DSA that is manifested by tensile tests. Furthermore. differences of DSA behaviors between two test materials of same specification were described and discussed with respect to the chemical composition.