Single-crystalline CuO nanoleaves with an average thickness of about 10 nm and with lateral sizes of hundreds of nanometers to several micrometers have been successfully synthesized by microwave-heating of an aqueous solution containing copper salt and sodium hydroxide. CuO nanoleaves were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and UV-vis absorption spectrum. The band gap of CuO nanoleaves was estimated to be approximate to2.13 eV, which shows significant blue-shift compared with that of the bulk.