Fatigue crack propagation (FCP) behavior of friction stir welded (FSWed) 6061-T651 Al alloy was examined with the fatigue crack growing either along the dynamically recrystallized zone (DXZ) at variable K or perpendicular to the DXZ at a constant K value of 7.5, 10 and 15 MPa√m, respectively. The FCP rates in the DXZ tended to be significantly lower than those in the PM particularly in low K regime. Compressive residual stress reducing effective K cannot be solely responsible for the enhanced FCP resistance. The fine recrystallized grains in the DXZ causing intergranular fatigue failure appeared to be detrimental to the FCP behavior of FSWed 6061-T651 specimen. The constant K fatigue test across the weld zone showed that substantial crack retardation occurred far beyond the heat affected zone (HAZ) at low K regime. The FCP behavior of FSWed 6061-T651 is discussed based on residual stress measurement and fractographic observation.