Objectives: Nanoparticles synthesis via green approach is a scorching theme of research nowadays as it minimizes the use of harmful chemicals. Not much work has been done on bio reduction of copper salts; therefore our present work was focused on green synthesis of copper oxide nanoparticles using leaf extract of Callistemon viminalis. Methods: Bio reduction method was used for preparation of copper oxide nanoparticles using Callistemon viminalis leaf extract. Synthesis was carried out at three different temperatures namely 4ºC, room temperature and 60ºC. Results: Completion of reaction occurred at 60ºC and reduced copper oxide particles were further characterized by ultra violet spectroscopy, dynamic light scattering technique, fourier transform infrared spectroscopy, X-ray diffraction and photoluminescence spectroscopy. The antibacterial evaluation of nanoparticles was done against standard bacterial strains Acinetobacter baumannii and Escherichia coli. Synthesized copper oxide nanoparticles were found to exhibit antibacterial activity in both pathogenic strains. Conclusions: Overall we can conclude that bio reduction is facilitated at 60ºC and it is rapid, simple, inexpensive and effective way of producing antibacterial copper oxide nanoparticles.