Novel Au core Au/Ag alloy shell particles denoted as Au@Au/Ag were prepared through the following two-step reduction method using branched Au nanoparticles as seeds. In the first step, branched Au seeds were prepared from an aqueous solution of an HAuCl₄/ammonium formate (AF)/poly(vinylpyrrolidone) (PVP) mixture. In the second step, AgNO₃ was reduced in N,N-dimethylformamide (DMF) in the presence of branched Au seeds and PVP. Then larger branched particles with smaller numbers of short branches were formed. On the basis of transmission electron microscopic (TEM), high-resolution (HR)-TEM, energy dispersed X-ray spectroscopic (EDS) and XRD data, crystal structures and their growth mechanisms were discussed. It was found that Au/Ag alloy shells were formed by simultaneous occurrence of melt of fine long branches in Au seeds and slow reduction of Ag⁺ to Ag⁰ during heating the DMF solution at 140 °C for 3 h. The fusion of fine Au branches was confirmed by observing shape change from fine branches to spherical particles on Au core particles in DMF without the presence of AgNO₃.