Co nucleation and growth on graphite were studied by scanning tunneling microscopy. Co atoms were found to adsorb preferentially on the B sites. Two- and three-dimensional Co clusters were observed, distributed uniformly on the graphite surface. The internal. structure of the Co cluster is quite disordered. Cluster evolution on the surface was observed in sequential images, suggesting that atom dissociation and incorporation may be one of the mechanisms for cluster migration. Co cluster-induced superstructures were observed to localize around the clusters on the surface. The patterns of some superstructures reflect graphite’s threefold symmetry. Thermal annealing of the sample resulted in Go-promoted etching of the graphite surface.