Quantitative and qualitative analyses on the dissolution behavior of various size Cu-Mg co-clusters near a fatigue crack tip of underaged Al-Cu-Mg alloy during cyclic loading were performed by means of transmission electron microscopy (TEM) and atom probe tomography (APT). Results showed that the driving force for dissolution of Cu-Mg co-clusters was increased with the increase of the co-clusters size. Therefore, Cu Mg co-clusters in smaller sizes were easier to dissolve. The number density of small Cu Mg co-clusters (20-50 atoms) increased and that of Cu-Mg co-clusters (50-200 atoms) reduced in the fatigue crack tip region during cyclic deformation. This was because that the large cluster could be repeated cut by moving dislocations to two or more parts of smaller clusters. Besides, the small clusters containing 20-100 atoms also might re-form after fatigue crack propagation (FCP) test in the fatigue crack tip region. The average equivalent radius of large Cu-Mg co-clusters (100-200 atoms) decreased from 1.38 nm to 131 nm during fatigue deformation in 170 degrees C/1 h sample. But there was no obvious difference of that in 170 degrees C/8 h sample. This result indicated that the dissolution of the large co-clusters in 170 degrees C/1 h sample was more violent than 170 degrees C/8 h sample. (C) 2016 Elsevier B.V. All rights reserved.