The present work employs Ocimum Basilicum extract as a bio-reducing and a capping agent to synthesize zinc oxide nanoparticles (ZnO NPs) at different temperatures (30 oC,60 oC, and 90 oC) in an efficient, simple, and environmentally friendly method (green synthesis), as well as their use in treating wastewater and antibacterial activity. The samples have a hexagonal structure confirmed by the X-ray Diffraction (XRD), and by using the Scherrer equation, the particle size was calculated. UV–Visible diffuse reflectance spectroscopy reveals the characteristic absorbance peak at 373 nm and gives the band gap nearly 3.23 eV for synthesized (ZnO NPs) due to the quantum confinement effect. Transmission electron microscopy (TEM) shows spherical-shaped particles. The products (ZnO-NPs) have a good effect on antibacterial activity against different types of bacteria. After 7 hours, the degradation rate of methylene blue (MB) dye by synthesized ZnO-NPs as a photocatalyst is 92.28 % from ZnO NPs synthesized at 30 oC.