The effect of particulates on the failure mechanism of an Al-Mg-Si alloy 6061 with 20% angular alumina particles was studied. Fracture toughness tests were conducted on compact tension peak-aged specimens. The interaction of the reinforcement phase with the crack was investigated by optical microscopy and scanning electron microscopy, both on the surface and in the mid-thickness of the fractured specimen. It is shown that the fractured particles ahead of the crack tip, in particular the larger particles, play an important role in the void-initiation phase of the fracture process. Particle size and aspect ratio determine the likelihood of fracture. Some differences in the failure mechanisms have been observed between the mid-thickness and the surface of the specimen because of the difference between plane strain and plane stress fractures.