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The effects of temperature and electrolysis potential on the alloying rate, structure, and mechanical strength for the Dy-alloyed Hastelloy C-276 samples, where Hastelloy C-276 is a Ni-based alloy containing Cr and Mo, were investigated in a molten LiF–CaF₂–DyF₃ (0.30 or 0.50 mol%) system at 1123–1323 K and a molten LiCl–KCl–DyCl₃ (0.50 mol%) system at 873 K. The microstructure was studied by electron microscopy and energy-dispersive X-ray spectrometry analyses, and the mechanical strength of the formed Dy-alloys was evaluated using punch tests. The alloying rate was influenced by the electrolysis potential and significantly by the temperature. Phase separation into DyNi₂ and Cr–Mo was observed, and a layered structure perpendicular to the depth direction was formed. The pitch of the layered structure was found to depend on the electrolysis potential, suggesting that the diffusion rate of Cr and Mo determined the structure. The Dy-alloyed samples electrolyzed at a more negative potential in the LiCl–KCl–DyCl₃ melt exhibited a higher mechanical strength. The Dy-alloyed samples obtained in the LiF–CaF₂–DyF₃ melt at 1223 K and 1323 K exhibited a low mechanical strength owing to the large grain size of the agglomerated Cr–Mo alloy phase.