Silver nanoparticles (AgNPs) with different shapes and sizes were chemically prepared and characterized by transmission electron microscope (TEM), UV-vis spectra and Fourier transmission IR (FTIR). Their antibacterial activities against gram positive bacteria (Staphylococcus aureus ATCC 6538, Staphylococcus epidermidis ATCC 12228) and gram negative bacteria ( E. coli ATCC 8739) were studied. Antimicrobial activities of Ag nanoparticles had been increased with their larger surface area to volume ratio. Best antibacterial activity was observed on using AgNPs prepared from sodium hypoborite (NABH4) as reducing agent and polyvinyl pyrrolidone (PVP) as stabilizer or protecting agent. It exhibit zone of inhibition about 25 mm against S. aureus ATCC 6538, 19 mm against S. epidermidis ATCC 12228 while the Cefoperazone antibiotic had inhibition zone of 15 mm. The same silver nano particles showed inhibition zone against E. coli ATCC 8739 of about 15 mm while the antibiotic had 12 mm inhibition zone. TEM revealed small size of silver nanoparticles (ranging from 1.5-3 nm) which stimulate biofilm production and aggregate within this biofilm. They bind closely to the surface of microorganisms causing visible damage to the cells, and demonstrating good self-assembling ability.