Using scanning tunneling microscope, we have investigated the formation of Au nanoclusters on self-assembled monolayers of octanethiol (CH3(CH2)(7)SH) molecules modified with octanedithiol (HS(CH2)(8)SH). By changing the modification condition and the amount of An deposition, it is found that the density and the size of the clusters can be controlled. The cluster-formation mechanism is discussed. (C) 2004 Elsevier B.V. All rights reserved.