Abstract: Results indicate positron annihilation lineshape parameter S and lifetime measurements for dislocation density change in A508cl.3 steel are sensitive. When fatigue damage N/N f≤ 10 %, both S parameter and mean lifetime increase with N/N f value. Results for decay constant measured by ultrasonic waves at frequencies of 5 MHz and 15 MHz agree almost with above mentioned ones. Microstructures of fatigued specimens were examined by TEM to verify the results obtained by NDE.