Scanning tunneling microscopy was employed to study Mo clustering and interaction with substrate on highly oriented pyrolytic graphite. Mo follows the island growth mode, and the Mo clusters are distributed uniformly on a graphite surface. Thermal effects promote the diffusion of edge atoms and result in smooth edge and shape changes of the clusters. The appearance of triangular Mo clusters reflects the metal-substrate interaction. Cluster diffusion on a graphite surface was observed and the value of the diffusion coefficient for small Mo clusters were estimated to be on the order of10−19cm2/sat room temperature. Cluster-induced electron scattering standing waves were observed on the graphite surface.