ZnO nanoparticles were successfully synthesized by using zinc dust waste derived from hot-dip galvanizing process as a starting materials via simple hydrothermal method. The zinc dust powder was dissolved in nitric acid, adjusted pH with NaOH and hydrothermally treated at 120-170°C in an oven. The resultant products were characterized for phase, surface area and morphology by X-ray diffraction (XRD), Brunauer-Emmet-Taller (BET) and Scanning electron microscopy (SEM), respectively. Moreover, the photocatalytic activity was investigated by a photodegradation of methylene blue solution under black light irradiation. It was shown that crystallinity of pure ZnO phase, was influenced by solution and hydrothermal treatment temperature. The experimental results exhibited that the crystallinity of ZnO significantly affected its photocatalytic activity. The highest photocatlytic activity was obtained from powders with average crystallite size of 51 nm and surface area of 13.4 m2/g prepared under pH12 and hydrothermally treated at 170°C for 6 h. Moreover, the synthesized ZnO powders performed a good antibacterial activity evaluated using a colony count method with Escherichia coli under the dark condition.