Nanocrystalline AA 6061 alloy reinforced with alumina (0, 4, 8, and 12wt.%) in amorphized state composite powder was synthesized by mechanical alloying and consolidated by conventional powder metallurgy route. The as-milled and as-sintered (573K and 673K) nanocomposites were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The peaks corresponding to fine alumina was not observed by XRD patterns due to amorphization. Using high-resolution transmission electron microscope, it is confirmed that the presence of amorphized alumina observed in Al lattice fringes. The crystallite size, lattice strain, deformation stress, and strain energy density of AA 6061 matrix were determined precisely from the first five most intensive reflection of XRD using simple Williamson–Hall models; uniform deformation model, uniform stress deformation model, and uniform energy density deformation model. Among the developed models, uniform energy density deformation model was observed to be the best fit and realistic model for mechanically alloyed powders. This model evidenced the more anisotropic nature of the ball milled powders. The XRD peaks of as-milled powder samples demonstrated a considerable broadening with percentage of reinforcement due to grain refinement and lattice distortions during same milling time (40h). The as-sintered (673K) unreinforced AA 6061 matrix crystallite size from well fitted uniform energy density deformation model was 98nm. The as-milled and as-sintered (673K) nanocrystallite matrix sizes for 12wt.% Al 2 O 3 well fitted by uniform energy density deformation model were 38nm and 77nm respectively, which indicate that the fine Al 2 O 3 pinned the matrix grain boundary and prevented the grain growth during sintering. Finally, the lattice parameter of Al matrix in as-milled and as-sintered conditions was also investigated in this paper.