Abstract: The fracture characteristics of precipitates in cast eutectic Al-Si alloys were studied during tensile processes by observing the fracture and precipitates.It is shown that the primary/eutectic Si cracks mainly through transcrystalline fracture owing to structural defects.Primary Si with a diameter size less than 2.0μm can block the propagation of secondary cracks.The blocky iron-rich phases crack through transcrystalline fracture,while cracks within the Chinese script iron-rich phases propagate along their primary dendrites.The copper-rich phases deflect the crack propagation and inhibit the nucleation of microcracks within the iron-rich phases.Increasing the Cu content neutralizes the harmful effects of the iron-rich phases to some extent and is of benefit to the mechanical properties.