The paper presents the use of positron annihilation lifetime spectroscopy (PALS) to characterize the thermally induced evolution of defects present in chemically synthesized ZnO nanoparticles using chemical bath deposition (CBD) method followed by annealing in the temperature range of 100 - 800ºC. The positron annihilation lifetime data were used to trace defect structure changes in the samples after annealing. The positron lifetime spectra were deconvoluted into three components in the annealing temperatures ranged from 400-800oC, while the annealed samples at 100-300oC were analyzed with four lifetime components. The calculated lifetime parameters of positron and positronium atoms allowed drawing conclusions about the degree and nature of defects of studied materials. In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. The UV- VIS Spectrophotometer was used to measure the absorbance of the synthesized ZnO nanoparticles