Self-assembled monolayers (SAMs) of alkanethiols on gold surface have been a popular technique in many aspects of scientific and technical research. Introduced in the review article is the formation mechanism of SAMs observed in ultrahigh vacuum and in diluted alkanethiol solutions by SPM (scanning probe microscope). The mechanism is generalized as two steps. At the initial stage, the alkanethiols appear stripes on gold surface, indicating a lying down orientation for the molecular chain. A first-order Langmuir adsorption isotherm is proposed. After saturation of the striped phase, the monolayer undergoes a phase transition, resulting in a (√3x√3)R30° where the molecular chains align ~30° from surface normal.