Silver nanoparticles of average sizes ~18-21 nm in diameter were prepared by a green approach via chemical reduction of silver nitrate (AgNO3) using Asiatic Pennywort and Bryophyllum leaf extracts as reducing and capping agents. The bio-reduced silver nanoparticles were characterized by UV-Vis spectroscopic, XRD and TEM techniques. The characteristic surface plasmon band of colloidal solutions of AgNPs synthesized from Asiatic Pennywort and Bryophyllum leaf extracts were found at 445 nm and 405 nm respectively. The results of XRD and SAED pattern showed that the biosynthesized AgNPs have a crystalline structure with cubic phase (fcc). The antimicrobial activities of the as synthesized AgNPs were investigated against gram negative bacteria Pseudomonas Fluorescens and gram positive bacteria Staphylococcus Epidermidis. It was observed that silver nanoparticles obtained from Asiatic Pennywort was more effective on gram positive bacteria Staphylococcus Epidermidis while AgNPs obtained from Bryophyllum was more effective on gram negative bacteria Pseudomonas Fluorescens indicating size dependent activity of AgNPs.