Abstract: Microstructures of in-situ reactive near-liquidus cast Al₂O₃/Al-Cu matrix composites were observed by optical microscope, and transmission electron microscope was used to examine the distribution and morphology of in-situ Al₂O₃ particles in the Al-Cu alloy matrix. The effect of in-situ Al₂O₃ particles on the formation mechanism of mierostructure of the near-liquidus east Al-Cu alloy was analyzed. It is shown that as-cast microstructures of the in-situ processed composites have a relatively uniform distribution of Al₂O₃ particles within 1 μm in average size and the Al₂O₃ particles have polygon morphology. The rose-flowered structure gradually fines and homogenizes with increasing the mass fraction of in-situ Al₂O₃ particles; while the mass fraction of Al₂O₃ particles is up to 5.3%, a fined and homogenized rose-flowered structure is obtained.