In the present work, Al-B_4C composite powders were fabricated using a mechanical milling process and its milling behaviors and mechanical properties as functions of B_4C sizes ( 100µm, 500 nm and 50 nm) and concentrations (1, 3 and 10 wt.%) were investigated. For achieving it, composite powders and their compacts were fabricated using a planetary ball mill machine and magnetic pulse compaction technology. Al-B_4C composite powders represent the most uniform dispersion at a milling speed of 200 rpm and a milling time of 240 minutes. Also, the smaller B_4C particles were presented, the more excellent compositing characteristics are exhibited. In particular, in the case of the 50 nm B_4C added compact, it showed the highest values of compaction density and hardness compared with the conditions of 100µm and 500 nm additions, leading to the enhancement its mechanical properties.