A simple method was developed to fabricate Au-Ag nanoparticle/graphene oxide nanocomposites (Au-Ag/GO) by using simultaneous redox reactions between AgNO3, HAuCl4 and GO. The Au-Ag/GO was characterized by x-ray photoelectron spectroscopy, transmission electron microscopy and energy dispersive x-ray spectroscopy. The GO nanosheets acted as the reducing agent and the support for the Au-Ag alloy nanoparticles. In addition, Au-Ag alloy nanoparticles with different shapes including core-shell-like, dendrimer-like and flower-like were obtained by simply modifying the concentration of the reactants and the reaction temperature. With no reducing or stabilizing agents added, the Au-Ag/GO nanocomposites show superior catalytic performance for the reduction of 4-nitrophenol and for the aerobic homocoupling of phenylboronic acid.