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This investigation is mainly aimed to study the influence of SiC and Al2O3 particles on the mechanical properties and damage evolution behaviors of an aluminum alloy Al-2618. Heat treatments for the composites are also studied to optimize their mechanical properties. The results of tensile tests show that SiC particulate reinforcement has advantages over Al2O3 reinforcement in both strength and ductility for the composites. T4 treatment is suggested for the composites rather than conventional peak-aging treatment (T6). T4 heat treatment with an additional of 0.6% pre-strain can result in same UTS and a 0.2% proof stress for the composites as high as T6 treatment but the final elongation under T4 treatment is larger than that under T6 treatment by more than 100%. Based on observation of damage evolution behaviors of the reinforcing particles, a theory that strength of the composites is mainly decided by the balance between reinforcing particles sharing load and making strain discontinuity in the matrix is proposed to interpret the test results. Their tolerance for large local strain at the interface, their high K1c and their low thermal expansion make SiC particles sharing much load and the better reinforcement over Al2O3 particles in respect to both strength and ductility of the composites.