The morphologies of cysteamine (CA) and propanethiol (PT) self-assembled monolayers (SAMs) on Au(111) prepared in H2O, ethanol, N,N,-dimenthylfonnamide (DMF), and toluene solution were imaged by scanning tunneling microscopy (STM). The STM measurements showed that the formation of domains like dendrites in CA SAMs was dependent on a type of solvent. On the other hand, a pit size in PT SAMs was controlled by a type of solvent without such a domain formation. These results clearly revealed that the solvent plays an important role in SAM formation.