In this work we have successfully prepared spherical and chain type silver nanoparticles of excellent size’s homogeneity, reproducibility and stability using tannic acid. The synthesized nanometallic structures were characterized for their shape, size, thermal stability and crystalline nature. The mechanism for the formation of Ag NPs and shape evolution of the chain structure has been vividly explained. Further, these NPs were found to exhibit significant antibacterial activity against gram negative Escherichia coli bacteria but to different extend indicating the influence of particles morphology on their antibacterial behaviours.