The properties of particle-reinforced aluminum composites depend on the microstructure and evolved uniformity distribution of particles in the matrix. The distribution of Al2O3 particles in the matrix and microstructure properties of varying volume fraction of particles up to 15% Al2O3 particle-reinforced Al metal matrix composites produced by the mechanical alloying technique was investigated. Alloying was performed in a planetary ball mill using a milling time varying from 0 h to 7 h, a ball-to-powder ratio of 10: 1, and a ball mill velocity of 400 rpm. The produced compositions were cold-pressed at 700 MPa with a single action and sintered at 600 degrees C for 3 h under Argon gas atmosphere. The relative density (both pressed and sintered), porosity and hardness of composites were also examined. The results of mechanical alloying processing were investigated with scanning electron microscopy (SEM), X-ray diffraction and laser particle size analyzer. SEM observations showed that relatively homogeneous distribution of Al2O3 reinforcement in the matrix could be obtained by mechanical alloying after 5 h. Moreover, the variation in hardness of the composites with alumina volume fraction and milling time was observed to be a strong function of the work hardening mechanism.