A green hydrothermal method was proposed for the synthesis of nanocrystalline ZnO2,using Zn5(CO3)2(OH)6 powder and 6 vol％ H2O2 aqueous solution as the starting materials.Characterization results from X-ray diffraction,Raman,high resolution transmission electron microscopy and selected area electron diffraction revealed that the products synthesized at 80-120℃ for 6-18 h were pure cubic phase ZnO2 nanocrystals.Room temperature photoluminescence spectra of the as-synthesized ZnO2 nanocrystals displayed a wide and strong emission band in the visible region of about 525-570 nm upon laser excitation at 325 nm,which may have originated from their surface state and other crystal defects.