Mechanical milling was used to synthesize Al nanostructured powder in a planetary ball-mill under argon atmosphere up to 20 h. The same process was conducted for Al-4 wt. % B4C nanocomposite powders to explore the role of nanosize reinforcements on mechanical milling stages. The results show that the addition of boron carbide particles accelerate the milling process, leading to a faster work hardening rate and fracture of aluminum matrix. Tensile of a nanostructured matrix of Al prepared via hot extrusion were investigated before and after incorporation of B4C nanoparticles. The results revealed a, higher yield strength and tensile strength for nanostructured Al matrix in contrast to the commercial coarse grained Al matrix. The same trend was also found to be valid for the nanocomposite samples with respect to the base matrix.