A pulsed laser emitting UV radiations generated by the third harmonic of Nd:YAG was applied for the synthesis of nano-structured ZnO2 and ZnO. For the synthesis of nanoparticles of ZnO2, a high-purity metallic plate of Zn target was fixed at the bottom of a glass cell, in the presence of deionized water mixed with oxidizing agent H2O2, under repeated laser irradiation. The optical properties, size and the morphology of the synthesized ZnO2 and ZnO by laser ablation was influenced strongly by post-annealing conditions which is not previously reported. By annealing ZnO2 at 200 degrees C for 8 h, the product (ZnO2) synthesized primarily was converted completely to ZnO. By variation of the annealing temperatures from 200 to 600 degrees C, the grain size of ZnO changes from 5 to 19 nm with a change in lattice parameters, the band gap and some other optical properties of nano-ZnO. (C) 2009 Elsevier B.V. All rights reserved.