Background:Most natural plants used in the synthesis of silver nanoparticles are limited to marine plants. To carry out applications, colloidal silver nanoparticles (AgNps) should have appropriate properties such as homogeneous shapes, small and narrow particle size distribution, and long time stability. This study aims to determine the effects of a variable process of AgNps mediated mangroveRhizophora stylosa(RS) leaf extract, and antibacterial activity.Methods: Synthesis of AgNps was carried out by stirring silver nitrate solution with aqueous extract. The characterization of AgNps was carried out using UV-Vis spectrophotometry, X-ray diffraction (XRD), Dynamic Light Scattering (DLS) zetasizer and Transmission Electron Microscopy (TEM). Evaluation of antibacterial activity was carried out onEscherichia coliandStaphylococcus aureus.Reaction conditions such as the concentration of metal ions (0.001 M, 0.005 M, and 0.01 M), extracts (1%, 3%, and 5% v/v), and the reaction time on the size and stability of nanoparticles were also explored.Results: The UV-Vis spectroscopy showed an absorption of colloidal AgNps in a wavelength range of 403–443 nm.  TEM analysis showed that as-synthesized AgNps were spherical in shape with a size range of 5–87 nm. The use of 0.001 M and 0.005 M of Ag+resulted in a smaller diameter than the synthesized AgNps, using 0.01 M Ag+, in the same extract concentration. The range of zeta potential was -24.9 mV to -27.7 mV. The as-synthesized AgNps were stable for more than one month. The XRD analysis showed four peaks, which were attributed to the face centered cubic crystal structure of metallic silver. The results of the silver nanoparticles synthesis showed good activity onEscherichia coliandStaphylococcus aureus, with an inhibition zone between 4.1–7.2 mm.Conclusions:The AgNps synthesized with RS leaf extract, which is a reducing agent, showed good potential as an antibacterial component.