The monolayer structures of perylene adsorbed on Ag(1 1 0) surface were studied by combining scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. Several superstructures (i.e. loose and compact dimer structures, compact trimer structures) were observed. DFT calculation results show that the interaction between molecule and Ag substrate is weak, the intermolecular interaction is comparable with that between molecule and substrate. The organic self-assembly on metal surface results from the balance between intermolecular interaction and molecule-substrate interaction. On Ag(1 1 0) surface, the dimer-alpha structure was found to be more stable than other structures. (C) 2009 Elsevier B.V. All rights reserved.