Several recent studies have attempted to impart [Au25(SR)18](-) with new properties by doping with foreign atoms. In this study, we studied the effect of copper doping on the electronic structure, geometric structure, and stability of [Au25(SR)18](-) with the aim of investigating the effect of foreign atom doping of [Au25(SR)18](-). CunAu25-n(SC2H4Ph)18 was synthesized by reducing complexes formed by the reaction between metal salts (copper and gold salts) and PhC2H4SH with NaBH4. Mass analysis revealed that the products contained CunAu25-n(SC2H4Ph)18 (n = 1-5) in high purity. Experimental and theoretical analysis of the synthesized clusters revealed that copper doping alters the optical properties and redox potentials of the cluster, greatly distorts its geometric structure, and reduces the cluster stability in solution. These findings are expected to be useful for developing design guidelines for functionalizing [Au25(SR)18](-) through doping with foreign atoms.