Copper oxide nanoparticles were successfully synthesized by mechanochemical reaction, which is a green, low cost, solvent free, rapid method and followed by calcining treatment. Copper acetate monohydrate and urea were used as reagents and the resulted precursor was calcined at 500 ?C for 2h in air. The scanning electron microscopy (SEM) revealed the formation of nanoparticles with an average size of about 86 nm. The Fourier transform infrared (FT–IR) spectrum and X-ray powder diffraction (XRD) pattern of the product confirmed all of reflections can be indexed to pure phase of CuO with a monoclinic crystal system. The diffuse reflectance spectrum (DRS) showed a band gap of 1.7 eV.