In2O3/CuO nanocomposites have been successfully synthesized by heating the precursor obtained via a mechanochemical reaction. The precursor and the nanocomposites were characterized using X-ray diffraction and scanning electron spectroscopy. The heat treatment of the precursor at 600℃ in air for 2h resulted in the formation of In2O3/CuO nanocomposites with an average crystal size of about 22nm, varying with the calcination temperature. The lattice parameter of the nanocomposites, lower than that of single In2O3, decreased with increasing the heat treatment temperature. The mechanochemical method is a novel, cheap and convenient technique suitable for large-scale synthesis of composite nanocrystallites. The mechanism of nanocrystallite growth is primarily investigated.