The current study examined the production of silver nanoparticles from silver nitrate (AgNO3) solution with the extract of Eucalyptus bicolor bark and the antimicrobial activity of the silver nanoparticles versus microorganisms. The silver nanoparticle was characterized by its color changes of extract, UV-Visible spectroscopy and SEM. The useful groups for alcoholic, extract were recognized by using Fourier transform infrared spectroscopy analysis (FTIR). In addition to we compared, between the biological effectiveness of alcoholic and nano extract of Eucalyptus bark against three types of pathogenic microbes: Staphylococcus. aureus, (Gram +ve), Pseudomonas, aeruginosa (Gram-ve), and yeast, Candida, albicans . Alcoholic extract offered the higher effect with Candida by inhibition zone of 14mm , S,aureus, of 13mm and pseudomonas of 10mm , While the nano extract has showed that the highest effect on growth of microorganism, Candida by inhibition zone of 20mm , staph of 19mm, and pseudomonas of 11mm .Furthermore,the combined effect of some antibiotics and alcoholic extract was studied against the same pathogenic microorganisms and the results showed that both :synergism and antagonism effects were seen, Carpenciline and the extract exhibited synergistic effect as evidenced by the increment of antimicrobial , activity with all bacteria(synergism) while Amoxicilline and the extract exhibited antagonistic effect against S.aureus (antagonism), and synergistic effect against P. aeruginosa Keywords: plants extract, silver nanoparticles, antimicrobial activities. Biosynthesis of Silver Nanoparticles Using Eucalyptus Bicolor