Using a combination of scanning tunneling microscopy (STM) and density functional theory calculations, we have studied the adsorption of tetracene on the Cu(1 1 0) (2 x 1) O substrate flat monolayer coverage the adsorbed molecules are in the flat-laying geometry with their long axis along the close-packed [0 0 1] direction of the substrate and a long-range ordered structure on the length scale up to 100 nm has been observed. DFT calculation results indicate a stronger interaction between tetracene molecules and Cu(1 1 0) substrate than Cu(1 1 0) (2 x 1) O substrate. The preferential adsorption sites have also been pointed out on both substrates. The observed wavelike structure is explained by the interdigitation of C H bonds of adjacent molecules. Crown Copyright (C) 2009 Published by Elsevier B. V. All rights reserved.