In this study, the ZnO nanopowders were synthesized by mechanochemical processing with a successive thermal decomposition reaction. The initial reactants mixture of zinc chloride and oxalic acid was milled from 30 min to 4 h and thermally treated for 1 h at 450 A degrees C. The influence of both, oxalic acid and the duration of milling, on the crystal structure, average crystallite size, average particle size, and the morphology of ZnO nanopowders were investigated. The qualitative analysis was performed using X-ray diffraction and Raman spectroscopy techniques. While the XRD analysis shows perfect long-range order and pure wurtzite structure of the synthesized ZnO powders, Raman spectroscopy indicates a different middle-range order; in addition, according to Raman spectra, it is found that lattice defects and impurities introduced in ZnO crystal structure depend on milling duration, in spite of applied thermal treatment. The particle size distribution was measured by laser diffraction, whereas the morphology of the powders was determined by scanning electron microscopy. Impurity contamination was studied using inductively coupled plasma analysis. The obtained results showed that the applied two-step method is appropriate for the synthesis of high crystalline ZnO nanopowders, with uniform spherical particles with diameter between 20 and 50 nm. Profound effect of aqueous solution of oxalic acid to prevent agglomeration of final product is presented.