The preparation of Au-on-Pd heteronanostructure (HNS) using citrate-stabilized polycrystalline Pd nanoparticles (NPs) as the seeds is described. The resulting Au-on-Pd HNS is characterized and it is found that the formation of Au-on-Pd HNS depends greatly on a ratio between Pd seeds and AuCl4− ions added and the optimal molar ratio is 10:1. If fewer AuCl4− ions are added (Pd/Au ratio is 100:1), the growth of Au NPs only occurs on part of the Pd seeds’ surface. The addition of more AuCl4− ions (Pd/Au ratio is 5:1) hinders the growth of Au NPs on the Pd seeds’ surface. To demonstrate the catalytic performance, the electrochemistry oxidation of ethanol and the reduction of p-nitrophenol by NaBH4 are chosen to examine the catalytic activity of Au-on-Pd HNS. Pd seeds, Au NPs, and poly(vinyl pyrrolidone) (PVP)-stabilized PdAu nanoalloy are used as the references for comparison. In the first reaction, the catalytic reactivity of Au-on-Pd HNS is better than that of corresponding pure Pd or Au NPs, while the opposite occurs for the latter reaction. The catalytic activity of Au-on-Pd HNS is much higher than that of PVP-stabilized PdAu nanoalloy.