Self-assembled monolayer(SAM)s of 1-octadecanethiol [ODT: CH₃(CH₂)₁₇SH] were formed on bare GaAs substrates by cleaving the substrates in a 1 mM ethanol. Both growth process and surface properties were investigated using surface force microscopy (SFM) and x-ray photoelectron spectroscopy (XPS). Dendritic shape of islands ∼20 nm in diameter appeared for 30 sec immersion. As the immersion time increased, these islands coalesced, connected each other, and finally the surfaces were uniformly covered with the monolayer after 1 day of immersion. High-resolution friction force microscope (FFM) images revealed a periodic structure that had two type of lines on the SAM. A structural model of the SAM, which successfully explained these experimental results, was presented. XPS results also indicated that SAM of ODT can protect a GaAs surface from oxidation. Furthermore, we demonstrated the formation of nanostructural SAM of ODT by use of A1GaAs/GaAs hetero-epitaxial substrates. SFM images revealed that ODT molecules selectively chemisorbed on GaAs surface regions resulting in the formation of nano-wires, 10 nm in width and ∼1 nm in height. The mechanism of the nanostructural SAM formation was also discussed.