ZnO nanocrystals have been synthesized by thermal decomposition of zinc oxalate without using any additives or solvent and characterized by XRD, BET, FE-SEM, DRS and HR-TEM techniques. The XRD analysis showed that the synthesized catalyst has hexagonal wurtzite type polycrystalline structure. HR-TEM analysis revealed the formation of ZnO nanocrystals and the increase in size of ZnO nanoparticle from 40 to 100 nm during the formation. FE-SEM indicates the presence of a nano chain like structures. Nano ZnO is more advantageous than commercial ZnO in the degradation of RO 4 because (i) it has maximum efficiency at pH 7, (ii) its antiphotocorrosion property is higher than commercial ZnO, (iii) it is stable and reusable. The mineralization of Reactive Orange 4 was also confirmed by TOC analysis. (C) 2011 Elsevier B.V. All rights reserved.