Electronic structures of Ag n Co − (n=6–8) were investigated by photoelectron spectroscopy and density-functional calculations. The local spin magnetic moments on Co were found to be 2.04, 1.48, and 0μ B for n=6, 7, and 8, respectively. A strong sp–d interaction in n=8 allows the Co 3d orbitals to form a closed electronic shell, whereas the Co 3d shells in n=6 and 7 remain open with finite spins under a weak sp–d interaction. The present result demonstrates significant size dependence of an sp–d interaction in a metal cluster containing a magnetic impurity.