Abstract: This investigation presents the interaction of dislocations and semicoherent precipitates (Ω phase) in an Al-Cu-Mg-Ag alloy. The study shows that the semicoherent precipitate is cut by dislocations during deformation. Conventional transmission electron microscopy (TEM) and high resolution TEM (HRTEM) observations demonstrate that shearing of precipitates by dislocations occurs by multiple cutting in the precipitate. The step height caused by cutting on the Ω precipitate is only several atomic layers of the Ω phase. The Ω phase is strengthened after shearing by dislocations. A strengthening mechanism is proposed on the basis of this observation. The strengthening mechanism is not only applicable to the Ω precipitate in Al-Cu-Mg-Ag alloys but also to the other semicoherent precipitates in other alloys.