Copper nanoparticles (CuNPs) were synthesized under ambient temperature by wet chemical method and evaluated their antibacterial activity by the means of well diffusion method. In this study, we have analyzed the effect of NaBH₄ concentration on the synthesis of CuNPs. Surface plasmon resonance (SPR) was occurred at 575 nm. Scanning electron microscopy micrographs shows CuNPs have spherical morphology with size range of 20-40 nm. Dynamic light scattering (DLS) showed the particles size increases with the concentration of reducing agent NaBH₄. As prepared, CuNPs, demonstrated antibacterial activity against E. coli, Staphylococcus aureus and Pseudomonas aeruginosa. The potential mechanism of antibacterial activity was discussed with diagrammatic representation.