Abstract In this study, we synthesize silver nanoparticles using Anredera cordifolia stem extract as a reducing agent. We evaluate the antimicrobial properties of these nanoparticles against gram negative bacterial strain (Escherichia coli) and gram positive bacterial strain (Staphylococcus aereus). It took less than 10 minutes after mixing stem extract with silver nitrate solution for silver nanoparticles to form, which is indicated by the change of the colour of the sample to yellowish brown, and its absorption to the visible light. The particles continue to form and complete the formation in one day. We found from UV-VIS spectra that 424 nm is the wavelength where the Plasmon resonance takes place. The FTIR spectra suggest that the silver nanoparticles formed contained alcohols and phenolic compounds as well as protein which are responsible for their formation and stabilization. Based on the inhibition zones, we found that the silver nanoparticles show antibacterial activity against both E. coli and S. aereus, while stem extracts alone do not show antibacterial activity against E. coli and S. aereus. In addition, antibacterial activity of the silver nanoparticles against both E. coli and S. aereus depends on the silver nitrate concentration. Analysis of variance indicates that the diameter of inhibition zone of 3 mM silver nitrate nanoparticles is significantly larger than that of 1 mM and 2 mM silver nitrate nanoparticles. We found no significant difference in diameter of inhibition zones between E. coli and S. aereus, which indicates a broad-spectrum antibacterial activity of the Anredera cordifolia stem extract mediated nanoparticles.