The present study examined the formation of single-walled carbon nanohorns (SWCNHs) dispersed with nanoparticles of a Pd alloy that is composed of one of nine elements-Au, Pt, Cu, Fe, Ni, Ti, Mo, W, and Nb-by a modified gas-injected arc-in-water (GI-AIW) method incorporating a hollow graphite anode into which wires of Pd and an alloying component were inserted to generate arc discharge. The following requirement to realize the formation of Pd-alloy nanoparticles dispersed in SWCNHs was recognized: the boiling points of the alloying component must be below the 5000 K arc plasma temperature. When this requirement is satisfied, the boiling point of this component could be used as a threshold to judge whether Pd should be enriched or diluted in the products formed by the arc discharge process. Particle size analysis revealed that nanoparticles of Pd alloy containing Au show significantly high dispersion. In contrast, alloys with Cu produce relatively large alloy nanoparticles. The tendency of the average size of the alloy nanoparticles produced by this method can be correlated by a simple equation, taking into account the ratio of boiling point to melting point, surface tension, and gas diffusivity of the alloying components.