Fatigue-crack-growth in an ultrafine-grained (UFG) Al-6%Mg-0.3%Sc alloy is investigated in conjunction with a precise analysis of the fracture surface. The comparison of the crack growth behavior of the UFG and ordinary polycrystalline materials has shown that the fatigue crack growth rate in the UFG alloy is higher than that in the coarse-grained material only in the near-threshold region. In the intermediate fatigue stage, propagation of the fatigue-crack in the UFG structure becomes insensitive to the grain size. At larger stress-intensity-factor-increments, K, the crack resistance of the UFG material is better than that of un-ECAPed specimen. Analysis of the surface features indicates that such inhibition of the crack growth in the UFG structure upon increasing K may be related to the gradual transition from intergranular- to transgranular mode of fatigue fracture.