Self-assembled monolayer of mesoscopic size with Tetrathiafuluvalene (TTF) skeleton has attracted because it is expected to have quantum size effect for perpendicular to the substrate as well as two-dimensional pi -electron interaction. Surface plasmon resonance (SPR) revealed that TTF-CH2SH monolayer is formed on Au (111) only at proper concentration. The presence of etch pits which is characteristic to chemisorbed SAMs was confirmed by scanning tunneling microscopy (STM). From those SPR and STM results, it has-been suggested that the molecular long axis of TTF-CH2SH was aligned nearly perpendicular to the substrate. in addition, cyclic voltammogram (CV) result indicates that TTF-thiolate SAMs on gold electrode are stable in repeated scans.