In this study, two new milling models based on the size of reinforcement materials were developed. Al2024-B4C nanostructured composite powders containing 0, 5, 10 and 20 weight percentage (wt.%) of B4C particles with two different particle sizes (d(50) = 49 mu m and d(50) = 5 mu m) as reinforcement material were produced by mechanical milling. The milling was carried out in a planetary ball mill for 10 h with a rotating speed of 400 rpm and ball to powder ratio of 10:1. The effects of reinforcement particle size and weight percentage on fracture and plastic deformation behavior of composite powders were investigated. The particle size, morphology and microhardness were used as the main criterions. The structural and mechanical evolutions of the mechanically milled powders were investigated using a scanning electron microscopy, a laser particle-size analyzer, an X-ray diffractometry and a microhardness tester. It was found that milling time as well as reinforcement size and reinforcement content were strongly affecting the particle size and microhardness of composite powders. The results showed that the coarse B4C particles accelerated the milling process due to cutting effect, while fine B4C particles accelerated the milling process due to embedding effect. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.