Zinc oxide nanoparticles (ZnO NPs) have gained a huge interest as a promising therapeutic alternative for antibiotics. In this study, the biosynthesis of ZnO NPs was performed using Aloe vera leaves extract in a remarkably simple approach. The biosynthesized ZnO NPs were detected by UV-Vis spectrophotometer at wavelength of 383 nm. The x-ray diffraction analysis indicated the hexagonal wurtzite of ZnO NPs with a median crystallite size of 16.7 nm. Transmission and scanning electron microscopy exhibited the hexagonal shape of ZnO NPs with a median particle size of ~ 20 nm. Fourier transform infrared analysis showed the mutual presence of phenolic compounds in Aloe vera leaves extract and ZnO NPs which reflected their principle role in the bio-reduction. From our perspective, the optimum conditions for the biosynthesis of ZnO NPs with smaller sizes included zinc nitrate concentration of 0.01-0.03M, Aloe vera extract concentration of 50%, pH=8, and 25-40 ̊C. The antibacterial impact of ZnO NPs was confirmed against both Gram-negative and Gram-positive bacterial isolates. Values of minimum inhibitory concentration (MIC) of ZnO NPs varied to be 2-4.1 mg/ml and 8.2-11 mg/ml for Gram-negative and Gram-positive bacterial isolates, respectively. Furthermore, ZnO NPs exhibited bactericidal impact on all tested bacterial isolates with minimum bactericidal concentration (MBC) of 2.1-4.7 mg/ml and 8.2-11.4 mg/ml for Gram-negative and Gram-positive bacterial isolates, respectively.