In this work we present the synthesis of Au@Ag bimetallic nanoparticles via seed-mediated method and their nonlinear optical properties. For the synthesis of nanoparticles, 18 nm gold seeds were synthesized by Turkevich method followed by the silver ion reduction with ascorbic acid. The nanoparticles dispersion was analyzed by UV-Vis spectroscopy and the spectra suggest the formation of spherical core@shell bimetallic nanoparticles. SEM images were obtained in order to corroborate this. The nanoparticles dispersions present negative and positive nonlinear absorption coefficient and negative refraction index, according to z-scan measurements. The nonlinear optical properties can be tailor modifying the thickness of shell.