Ductility of aluminum alloy was studied in correlation with the fracture surface structure observed by electron microfractography. The results obtained are as follows:In 1100 and 3003 alloys, the fracture surface was characterized by deep and large dimples, nucleated at lump shaped inclusions. However, about 10% of inclusions were associated with the dimple nucleation. The dimple density was correlated with the number of large inclusions rather than with the density of whole inclusions. The brittleness in the cold worked state was due to dimple initiation occurred easily.In an Al-4%Cu alloy, the most ductile state was produced by the G. P. zone formation; very large dimples were observed on the fracture surface. G. P. zones were, thus, considered to contribute to hardening of the matrix but not to the dimple nucleat on, while inclusion particles initiated the dimples. The ductility was the lowest when the Θ' precipitation occured; very fine dimples were present on the fracture surface. A large number of weak Θ' precipitates acted as the dimple nuclei. Very fine dimples, nucleated at Θ particles, were observed at the over aged state. The small number of large precipitates resulted in the recovery of ductility.