CuO nanoparticles with an average size of ca. 4nm have been successfully prepared by microwave irradiation, usingcopper (II) acetate and sodium hydroxide as the starting materials and ethanol as the solvent. The CuO nanoparticlesare characterized by using techniques such as X-ray powder diffraction, transmission electron microscopy, selected areaelectron diffraction, X-ray photoelectron spectroscopy and UV–Visible absorption spectroscopy. The as-prepared CuOnanoparticles have regular shape, narrow size distribution and high purity. The band gap is estimated to be 2.43 eVaccording to the results of the optical measurements of the CuO nanoparticles.