Synthesis of nano-material with the controlled size and shape is the prime concern of research in nanotechnology. In this study, the synthesis of silver nanoparticles (Ag NPs) were carried out using Plectranthus amboinicus leaf extract as reducing agent. The synthesized Ag NPs was characterized by using X-ray diffraction, scanning electron microscope, Fourier transform infrared spectroscopy and UV-vis spectroscopy. We further investigated the catalytic property of the biosynthesized Ag NPs by degradation of methylene blue (MB) in the presence of sodium borohydride. Compared with commercial Ag NPs, the proposed Plectranthus amboinicus leaf extract-deduced Ag NPs exhibited a much higher catalytic performance due to the well dispersibility caused by the attachment of biomolecules on the Ag NPs surface.