The effect of dynamic strain aging on cyclic stress response and fatigue resistance of ASME SA508 C1.3 forging steel for nuclear reactor pressure vessels has been evaluated in the temperature range of room temperature to 500 degrees C. Total strain ranges and strain rates were varied from 0.7 to 2.0% and from 4X10(-4) to 1X10(-2) s(-1) respectively. The cyclic stress response depended on the testing temperature, strain rate, and strain range. Generally, the initial cyclic hardening was immediately followed by cyclic softening at all strain rates. However, at 300 degrees C, the operating temperature of nuclear reactor pressure vessels, the variation of cyclic stress amplitude showed the primary and secondary hardening stages dependent on the strain rate and strain range. Dynamic strain aging was manifested by enhanced cyclic hardening, distinguished secondary hardening, and negative strain rate sensitivity. A modified cell shuttling model was described for the onset of the secondary hardening due to the dynamic strain aging and it was in good agreement with the experimental results. Fatigue life increased with increase in strain rate at all testing temperatures. Specifically the fatigue life was longer at the dynamic strain aging temperature. Further, the dynamic strain aging was easy to initiate the crack, while crack propagation was retarded by crack branching and suppression of plastic zone, hence the dynamic strain aging caused the improvement of fatigue resistance.