10.22376/ijpbs.2019.10.1.p1-12 Volume 12 Issue 1 2021 (January-March) Nanoparticles have gained utmost significance due to their unique physical and chemical properties such as higher surface area, lower melting point, specific magnetic property, mechanical strength and specific optical properties. The green biological synthesis method used in the present study is a non-toxic alternative to the conventional physical and chemical nanoparticles synthesis and could be suitable for large scale production biologically. This method could be appropriate for various prospective treatments. The unique antimicrobial properties of silver (Ag) nanoparticles gained high attention in the field of nanotechnology. lessThan i greaterThan Calotropis gigantea lessThan /i greaterThan has wide medicinal applications, hence it is utilized for Ag nanoparticles synthesis. In the present study, synthesis of silver nanoparticles is reported by bioreduction of lessThan i greaterThan Calotropis gigantea lessThan /i greaterThan extract using silver nitrate at room temperature. The synthesized nanoparticles were then characterized and evaluated for antimicrobial and antioxidant activity. Silver nanoparticles were characterized in terms of synthesis, size distribution and microscopic evaluation by UV-Visible spectroscopy and Transmission Electron Microscope (TEM). The change of color from light brown to dark brown confirmed the biosynthesis of silver nanoparticles. Nearly spherical nanoparticles with a majority of particle size less than 50 nm were confirmed. X-ray diffraction (XRD) pattern confirmed the existence of silver in the sample. The XRD diffraction patterns revealed four diffraction peaks at 38.45°, 44.83°, 64.7° and 78.06° indexed to 110, 111, 211, 222 planes fcc. lessThan i greaterThan In vitro lessThan /i greaterThan dose dependent free radical scavenging activity of biosynthesized silver nanoparticles was analyzed using 1, 1-Diphenyl-2-picrylhydrazyl (DPPH) and 2,2'-azino-bis [3-ethylbenzothiazoline-6-sulphonic acid] salt (ABTS). The synthesized nanoparticles have the tendency to prevent cell/tissue damage occurring because of the oxidative stress. The antibacterial activity was performed against lessThan i greaterThan B. subtilis lessThan /i greaterThan , lessThan i greaterThan S. aureus lessThan /i greaterThan , lessThan i greaterThan E. coli lessThan /i greaterThan and lessThan i greaterThan S. typhi lessThan /i greaterThan . These nanoparticles revealed higher antibacterial activity against lessThan i greaterThan S. typhi lessThan /i greaterThan and lessThan i greaterThan E. coli lessThan /i greaterThan as compared to lessThan i greaterThan S. aureus lessThan /i greaterThan and lessThan i greaterThan B. subtilis. lessThan /i greaterThan The minimum inhibitory concentrations were in the range 6.6 nM and 13.2 nM. Bhawna Nohwal, Nitish Kumar and Reeti Chaudhary Calotropis gigantea, Silver nanoparticles, Anti-microbial, UV-Vis spectroscopy, Transmission electron microscopy, X-ray diffraction. 72-78