The biological synthesis of silver nanoparticles (Ag/NPs) at a different condition using hot water seed extract of Calendula officinalis as reducing agent is reported. The as-prepared Ag/NPs were characterized by UV–vis spectroscopy; transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy analysis. UV–visible spectrum indicated a good absorption peak at 440 nm relative to Ag/NPs. TEM result shows a spherical cubic structure with an average size ranging from 5–10 nm for silver nanoparticle. Stability of bio-reduced silver nanoparticles was analyzed using UV–vis absorption spectra. Result shows that the Ag/NPs were found to be amazingly stable even after 4 months.