Eco-friendly synthesis of nanoparticles (NP) using plant extract has become very popular among nanoscience researchers. Here, we report synthesis of silver nanoparticles (AgNPs) using leaf extracts of Helianthus annuus and Mentha longifolia. These NPs were characterized by UV-Vis, TEM, XRD, Fluorescence and FTIR spectroscopy and were found to have roughly spherical morphology with average particle size of 10-25 nm in diameter. Thermal stability of the nanomaterial was estimated using TGA/DSC analysis. Antimicrobial activity of these AgNPs was also screened against Escherichia coli and the antibacterial effect was also monitored using SEM. The AgNPs were found to be better antimicrobial agent compared to standard antibiotics like ampicillin or kanamycin.