Abstract: Hybrid fiber laser and pulsed arc welded aluminum alloys via sound polishing were prepared and then three dimensional in situ fatigue imaging of welded joints was conducted by the high resolution synchrotron radiation X-ray computed microtomography (SR-μCT). Experimental results show that fatigue cracks are frequently initiated and propagated from the broken pores at the surface or sub-surface of hybrid welds, presenting a typical I mode semi-elliptical and quarter ellipse or circle profile. It is also found that the inner fatigue crack growth is faster than the surface crack with significant local acceleration and arrest behaviors together with highly nonlinear spatial propagation. Moreover, the longer the characteristic length of initial crack, the shorter the fatigue life of hybrid welds. Besides, there is a good agreement between the prediction of fatigue crack morphology based on Newman method and experimental results.