Controlled mechanochemical reduction of NiO has been carried out by room-temperature ball milling under H2 atmosphere. During the milling, a gradual conversion of NiO into Ni occurs, with no intermediate phases being formed. The amount of Ni and its crystallite size can be tuned by the ball-milling parameters, leading to the formation of Ni-NiO (metal-oxide) nanocomposites. The initial microstructure of the NiO powder is shown to play an important role in the reduction rate and on the final microstructure of the nanocomposites formed. The results indicate that the mechanically induced defects have a strong influence on the kinetics of the reduction process.