In this work the fatigue crack growth properties of friction stir welded butt joints are evaluated. Fatigue Crack Growth (FCG) tests have been carried out on two particle-reinforced aluminium alloys (AA6061/Al2O3/20p and AA7005/Al2O3/10p). FCG properties have been evaluated at the centre and at the side of the weld, respectively. The results are compared with FCG properties of base materials. The role of alumina in particles inside the matrix is evident in the threshold region: this can be explained in terms of Roughness Induced Crack Closure.