Adsorption and electronic structure of octithiophene (8T) molecules on Cu(111) and Au(111) surfaces are investigated using scanning tunneling microscopy (STM) and spectroscopy (STS) at room temperature. We find a large difference in adsorption behavior of 8T molecules on the two surfaces. At the initial stage of adsorption, 8T molecules are stabilized in the form of molecular chain on a terrace of Cu(111), whereas neither such chain structure nor isolated 8T molecules have been observed on a terrace of Au(111). By increasing the amount of adsorbed molecules, a disordered monolayer film is formed on Cu(111) while a well-ordered monolayer film is formed on Au(111). From the spectroscopic investigations using bias-dependent STM images and STS spectra and by comparing the data with theoretical calculations, it is found that the electronic property of 8T molecules in the molecular chain on Cu(111) is different from that of a free-standing 8T molecule while that in the monolayer film on Au(111) keeps original character of the free-standing 8T molecule. The present study shows that adsorption of 8T molecules on Cu(111) results in a formation of adsorption-induced states near the Fermi level. Copyright © 2012 American Scientific Publishers. All rights reserved.