Abstract: In order to investigate the fatigue behavior and failure mechanism in super long life regime, ultrasonic fatigue tests were performed on 5052 aluminium alloy welded joint. The results show that the fatigue strength of welded joint descended about 73.3% as compared to the base metal with the same fatigue life. Fatigue failure occurred in the very high cycle fatigue regime. Fatigue crack initiated from the welding defects(pores) from the observation of SEM. To clarify the effect of pores on thecrack initiation and propagation of welded joint, finite element method was used to calculate the stress concentration and stress intensity factor around the pores. Finally, the characteristics of fatigue crack initiation and propagation with welding defects were discussed.