The main challenge in the production of metal matrix composites reinforced by carbon nanotubes (CNTs) is the development of a manufacturing process ensuring the dispersion of nanoparticles without damaging them, and the formation of a strong bond with the metallic matrix to achieve an effective load transfer, so that the maximum reinforcement effect of CNTs will be accomplished. This research focuses on the production by powder metallurgy of aluminum and nickel matrix composites reinforced by CNTs, using ultrasonication as the dispersion and mixture process. Microstructural characterization of nanocomposites was performed by optical microscopy (OM), scanning and transmission electron microscopy (SEM and TEM), electron backscattered diffraction (EBSD) and high-resolution transmission electron microscopy (HRTEM). Microstructural characterization revealed that the use of ultrasonication as the dispersion and mixture process in the production of Al/CNT and Ni/CNT nanocomposites promoted the dispersion and embedding of individual CNT in the metallic matrices. CNT clusters at grain boundary junctions were also observed. The strengthening effect of the CNTs is shown by the increase in hardness for all nanocomposites. The highest hardness values were observed for Al/CNT and Ni/CNT nanocomposites, with a 1.00 vol % CNTs.