ZnO nanoparticles have been synthesized via chemical and green routes to draw a contrast between their structural and optical properties. The chemically synthesized nanoparticles have been synthesized via the precipitation method. The nanoparticles have been annealed at 300 ℃ and 400 ℃. However, green synthesized nanoparticles have been synthesized using garlic extract and then annealed at 400 ℃. X-ray diffraction results confirmed the formation of hexagonal wurtzite ZnO nanoparticles. The annealing of the nanoparticles enhances their crystallinity. The UV–vis spectra show the characteristic peak of ZnO nanoparticles, and the bandgap of the nanoparticles decreased with annealing. In the optical analysis, the refractive index and Urbach energy of the nanoparticles have been calculated. The presence of chemical bonding has been confirmed using Fourier transform infrared spectroscopy. The results show that the annealed nanoparticles formed from garlic extract are structurally better than the annealed nanoparticles synthesized via the chemical route.