Abstract : In this work, we present a green synthesis of silver nanoparticles at different UV irradiation times, stabilized in aqueous solution by a natural polysaccharide: alginate. Our objective is twofold: to synthesize these nanoparticles in the above-mentioned conditions, to characterize their morphology and size and to use them for elaboration of a nanocomposite film based on chitosan. From UV-vis spectrophotometry measurements, we have shown that the silver nanoparticles synthesized are of spherical shape or like a sphere . The appearance of the resonance spectra of the expanded surface plasmons, assume that the diameters of the nanoparticles are high. These results have been confirmed by dynamic light scattering measurements: diameters vary from a few nanometers to a few microns. We have developed nanocomposite films with chitosan from these prepared nanoparticles and we studied the interaction of the stabilizing agent alginate, with chitosan by Fourier transform Infrared Spectroscopy. The evolution of the IR spectra before and after formation of the polyelectrolyte complex alginate / chitosan confirms the reaction between them and also confirms the existence of silver nanoparticles in the films. The X-ray diffractograms show characteristic peaks of the crystalline planes of silver. Finally, we clearly demonstrated the antibacterial effect of the prepared nanocomposite films for Gram + and Gram – bacteria cells. Key words Alginate, silver nanoparticules , green synthesis, chitosan, nanocmposite films.