Biosynthesis of nanoparticles by plant extracts is currently under exploitation. Plant extracts are very cost effective and eco-friendly and thus can be an economic and efficient alternative for the large-scale synthesis of nanoparticles. Here we report extracellular biosynthesis of gold nanoparticles using flower extracts of Plumeria alba as reducing agent. The complete reduction of auric chloride was observed after 48 h of reaction at 30 ºC. The characteristic color changes from pale yellow to dark brown during the formation of gold nanoparticles in the reaction due to their specific properties (Surface Plasmon Resonance) was observed. The flower extracts acts as reducing as well as encapsulating agent for the gold nanoparticles. The UV-Vis spectroscopy and transmission electron microscopy (TEM) was used to characterize the obtained gold nanoparticles. The UVvisible spectra indicate a strong Plasmon resonance that is located at ~550 nm. The TEM images show that obtained samples have spherical morphology with two different size particles smaller particles with 20-30 nm size and bigger one with 80-150 nm particles. The antimicrobial activities of these gold nanoparticles against different microorganisms were also studied.