Abstract: Behavior of fatigue crack initiation and propagation in GH536 superalloy welded joint was real-timely observed by in-situ fatigue experiment. And the mechanism of fatigue crack initiation and propagation was analyzed. During fatigue cycle loading, dislocations move along slip bands and pile up in front of grain boundaries. Grain boundaries hinder the movement of dislocations, eventually causing slip-bands separation and transgranular fatigue crack initiation. At the beginning of crack propagation, crack tip extends in the pattern of "Z" under the alternate effect of single slip systems; and then crack tip extends perpendicularly to the loading direction under the effect of main stress. Grain boundaries and carbides hinder the extending of fatigue cracks.