Zinc oxide (ZnO) nanoparticles were synthesized by modified hydrothermal method at 160^oC under different reaction times. The analysis of the XRD pattern lines is considered a simple and effective method in estimating the crystallite size and the corresponding lattice strain. X-ray patterns were analyzed using Scherrer method and Williamson–Hall treatment to determine the crystallite size and the lattice strain. The XRD pattern confirmed the presence of crystalline hexagonal wurtzite ZnO nanoparticles with average crystallite size in the range 30 –36 nm with different reaction times. ZnO nanoparticles were also characterized by high resolution transmission electron microscopy (HR-TEM), UV-visible spectroscopy and thermal analysis.