Co3O4 and CuO nanoparticles have been successfully synthesized via an uncomplicated mechanochemical solvent-free method. Their crystalline structures and their average diameters between 5 and 8 nm, were determined fromXRD patterns and byHR-TEMimages. Particle sizeswere controlled through the concentration of starting materials and heating temperature. The UV-visible electronic absorption, raman spectroscopy and electronic paramagnetic resonance spectra of Co3O4 and CuO nanocrystals show clear evidence of the quantum size effect. The mechanochemical method as a simple process provides economically viable route for largescale nanomaterials production.