The antimicrobial effects of various metal ions or salts are well known, but the effects of Cu nanoparticles on microorganisms and antimicrobial mechanism have not been revealed clearly. The objective of the present study is to test the antimicrobial activity of copper nanoparticals. Linoleic acid capped copper nanoparticles have been prepared through the reduction of cupric ions by ethanol. The structure of these nanoparticles have been investigated using transmission electron microscopy, which shows that average size of these nanoparticles is 6 nm with a narrow size distribution. The X-diffraction study shows that the nanoparticles are crystalline and composed of face-centered cubic (fcc) copper. To elucidate the antimicrobial activity of copper nanoparticals, its inhibitory tests were performed on different bacterial strains. The antimicrobial activity of copper nanopartical was found to be much effective on S. aureus, E. coli and S. basillus . These results suggest that Cu nanoparticles can be used as effective growth inhibitors in various microorganisms, making them applicable to diverse medical devices and antimicrobial control systems.