An SEM study was made of the micromechanism of fracture in SiC particle-reinforced 6061 aluminium composites. The fracture toughness tests on the composites with SiC volume fractions of 0%, 10% and 20% were conducted on single-edge notched sheet specimens. Both qualitative observations of void nucleation at the notch root of the composite samples and quantitative measurements of crack profiles are made to assess the special role of the particle effects in these composites. The results are discussed with respect to the micromechanism of particle breakage and interface debonding and their effect on the nucleation and propagation of microcracks. Two kinds of void are defined to explain the facts that Al/SiC is brittle macroscopically and ductile microscopically. The direction of crack propagation in Al/SiC and the microstructure in the tip region of the crack are also studied with these results.