The effect of milling time on the microstructure and mechanical properties of Al and Al-10 wt.% Mg matrix nanocomposites reinforced with 10 wt.% Al2O3 during mechanical alloying was investigated. Steady-state situation was occurred in Al-10 Mg/10Al(2)O(3) nanocomposite after 20 h, due to solution of Mg into Al matrix, while the situation was not observed in Al/10Al(2)O(3) nanocomposite at the same time. For the binary Al-Mg matrix, after 20 h, the average crystallite size was 25 nm. Up to 20 h, the lattice strain increased to about 0.67% for Al-Mg matrix. By milling for 10 h the large increase in microhardness (190 HV) for Al-Mg matrix nanocomposite was caused by grain refinement and solid solution formation. From 10 to 20 h, slower rate of increasing in microhardness, and at 20 h micro hardness reach (201 HV). (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.