ZnO nanoparticles were synthesised by a one-step mechanochemical process using ZnSO4 and NaOH as reactants with NaCl acting as a diluent. A short milling time of 30 min was required for complete reaction. The effects of oxygen vacancies and the milling time on the photocatalytic activities of the prepared nanoparticles were investigated. The structural and morphological properties of the nanoparticles were evaluated by various analytical methods, including x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The particle size of the ZnO nanoparticles without heat treatment was approximately 50 nm but was increased up to 80 nm after heat treatment at 400°C. Optical properties such as the optical band gap and the photocatalytic activity were investigated by photoluminescence and UV-vis spectroscopy, as well as by photocatalytic experiments.