Formation of monodispersed Co nanoclusters on a single-crystal dielectric film at room temperature is reported. A remarkably narrow size distribution with the average size of atoms has been obtained. We have confirmed that the average size of Co nanoclusters is independent of the Co coverage and the cluster areal density linearly proportions to the Co deposition amount even at high coverages. Also, we have found that Co nanoclusters deposited on are thermally stable with respect to cluster aggregation/coalescence. We propose that this novel phenomenon is a quantum size effect, manifested by local energy minima in the electronic shell structure of Co quantum dots.