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The fatigue crack propagation lives of low alloy steel in the heat affected zone (HAZ) and fusion zone (FZ) were estimated based on the frequency response changes in ultrasonic fatigue, which varied with the crack size during fatigue. Results showed that more than 99% of the total fatigue life was occupied by the crack initiation process for HAZ in gigacycle fatigue. However, this number was scattered for FZ specimen owing to the crack propagation from welding defects directly. A nonlinear fatigue model based on natural frequency changes was developed, which described fatigue damage for HAZ and FZ specimens successfully.