A facile synthesis of flower-shaped porous Au-Pd alloy nanoparticles with ascorbic acid as a reductant and PVP as a stabilizing agent is presented. The alloy nanoparticles were prepared from the aqueous solutions of HAuCl4/K2PdCl4 mixtures in molar ratios of 3: 1, 1: 1, and 1:3. The size, structure, optical properties, and composition distribution of the synthesized Au-Pd alloy nanoparticles were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, UV-vis spectroscopy, and X-ray diffraction. The experimental results for the bimetallic systems and the physical mixtures of individual monometallic nanoparticles revealed that unstable small nanoparticles aggregate into the three-dimensional flower-shaped nanoparticles and the prepared nanoparticles are Au-Pd alloys. The surfaces of Au-Pd alloy nanoparticles were characterized by cyclic voltammetry measurement in 0.1 M HClO4 and surface-enhanced Raman scattering spectra of 1,4-phenylene diisocyanide adsorbed thereon. All alloy nanoparticles have a Pd-enriched surface.