Nanoparticles of Ru-Ni with a core-and-shell structure were synthesized as potential catalysts for fuel cells and other applications in a single-step spray-pyrolysis process at 700 degrees- 800 degrees C. The majority of the core consists of ruthenium, while the shell is predominately composed of nickel. Bimetallic nanoparticles with a core-and-shell structure are being considered as new and promising catalysts with enhanced catalytic activity, better stability, and higher resistance to contaminants for fuel cells and other applications. An aqueous precursor containing ruthenium chloride and nickel chloride was nebulized by an ultrasonic atomizer to generate an aerosol. Droplets were subsequently decomposed to form uniformly distributed Ru-Ni bimetallic nanoparticles, then deposited on a substrate. Atomic fractions and melting temperatures are expected to play a crucial role in the formation of core-and-shell structures.