The self-assembled (SA)molecular monolayers of a 3-aminopropyltrimethoxysilane (3-APTS) on a silicon (111) surface, and the effects of ultraviolet (UV) pre-treatment for substrates on the assembling process have been studied using XPS and atomic forcemicroscopy (AFM). The results show that the SA 3-APTS molecules are bonded to the substrate surface in a nearly vertical orientation, with a thickness of the monolayer of about 0.8–1.5 nm. The SA molecular monolayers show a substantial degree of disorder in molecular packing, which are believed to result from the interactions of amine tails in the silane molecules used with surface functionalities of the substrates, and the oxygen-containing species from the ambient. UV irradiation for silicon substrates prior to the self-assembly reaction can enhance the assembly process and hence, significantly increase the coverage of themonolayer assembled for the substrates.