Quasi-spherical silver nanoparticles (AgNPs) were synthesized by the extracts of different medicinal plants including Syzygium aromaticum, Cornus officinalis, Prunus mume and Sanguisorba officinalis, and were characterized by UV-Visible spectroscopy, transmission electron microscopy and X-ray diffraction. When pH values of the extracts were adjusted to 10, the biosynthesized AgNPs were well-dispersed and highly stable, and are relatively small in size. Thus obtained AgNPs were found to demonstrate good antibacterial activity against Escherichia coli and Staphylococcus aureus, with minimum inhibition concentrations of 1.69 and 3.38 μg/mL required, respectively.