Abstract: The influences of mechanical mis-match on crack growth direction and crack growth rate were studied based on the analysis of fatigue crack growth behaviors in electron beam welded joints of supperalloy, The results showed that the fatigue crack growth of electron beam welded joints was instable due to the influence of mechanical heterogeneity. The fatigue crack located in welded seam of over matched joints deviated from its original growth direction, then spanned weld zone and base metal at a mixed mode while propagating. The effective growth driving force at the tip of fatigue crack reduced with the reducing of effective stress intensity factor owing to crack deviation of weld zone,which led to the fatigue crack rate deduced. For this reason, the overmatched joints have the capacity of protecting fatigue crack joints from fast growth. Fatigue crack was strongly affected by size and symmetrical characteristic of plastic zone at crack tip, so the integrity of welded structure containing fatigue crack mainly depended on the toughness of lower strength zone.