Samples of 1560 (Al-Mg-Mn) and 1960 (Al-Zn-Mg-Cu) alloys have been used to investigate the nature of the effect of grain size and superplastic treatment on the strength of aluminium alloys. The observed increase in the work needed for crack formation with the transition from coarse-grained (CG) to fine-grained (FG) structure is connected to a greater homogeneity of the plastic deformation in the material volume. This leads to a reduction in local stress concentrations at the sites of preferential crack initiation. The easier crack growth in FG alloys is mainly caused by a reduction in the energy for plastic deformation at the head of a long crack and also for the formation of free fracture surfaces.