We have succeeded in observing Pd nano-clusters, catalytic prime elements, on a GaN(0 0 0 1) surface by a scanning tunneling microscope for the first time. After the sample was reused, we found that nano-clusters (width: 11 nm, height: 2.2 nm) existed on the surface which still kept the catalytic activity, resulting that the neutral Pd atoms formed the nano-cluster. Moreover, the S-termination contributed to the formation of the dense and flat structure consisting of the Pd nano-clusters. (C) 2011 Elsevier B.V. All rights reserved.