Metal oxide nanoparticles have gained significant attention in recent years due to their unique properties and potential applications in various fields. However, the size of these nanoparticles plays a crucial role in determining their properties and applications. In this study, we investigated the size determination of metal oxide nanoparticles using UV-Vis spectroscopy. We used zinc oxide (ZnO) nanoparticles as a model system and demonstrated that the band gap energy of the nanoparticles can be used to determine their size. The results show that the band gap energy increases as the particle size decreases, consistent with the Brus equation. We also demonstrated that the UV-Vis absorption spectra can be used to determine the size of the nanoparticles. The Summary of this study have important implications for the development of new nanoparticles with specific properties and highlight the potential of UV-Vis spectroscopy as a tool for size determination of metal oxide nanoparticles.