ZnO nanoparticles (NPs) have been synthesized by precipitation method and auto-ignition method from zinc nitrate hexahydrate. The powder was characterized by thermal analysis (TGA-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Ultra violet- Visible optical absorption (Uv-Vis), and photoluminescence spectroscopy analyses (PL). XRD patterns for both catalysts showed that ZnO nanoparticles have hexagonal unit cell structure. SEM and TEM pictures detect the morphology and particle size of the as-prepared ZnO NPs. The UV-Vis absorption spectrum shows ZnO nanoparticles prepared by precipitation method has an absorption band at 385 nm while by auto-ignition method has an absorption band at 355nm. PL spectra proved that ZnO prepared by auto-ignition method was the most active one. This indicated a minimum recombination rate. These ZnO nanoparticles can be used in different applications due to their great optical properties.