ZnO nanoparticles of different particle sizes were synthesized via the hydrothermal method by varying the autoclave reaction temperature from 100 ◦C to 200 ◦C. The shape and particle size of the produced ZnO nanoparticles were investigated by X-ray diffraction (XRD), UV–visible spectrophotometer, scanning electron microscopy (SEM), transmission electron microscope (TEM), and laser-induced breakdown spectroscopy technique (LIBS). These studies showed that the prepared samples had high purity crystalline structure with wurtzite hexagonal ZnO and its crystallinity increases as the reaction temperature increases. Its absorption spectrum showed that the maximum absorption intensity was shifted to a higher wavelength, followed by increasing their particle sizes as the reaction temperature increases. Its morphological images showed increases in roughness and decreases in particle sizes as the reaction temperature increases. The intensity of the characteristic Zn peak of LIBS spectrum was decreased due to increasing of their particle size as the reaction temperature increases. Finally, the dielectric behavior of the prepared samples was studied revealing the dependence on their particle sizes.