The photoreactivity of an n-octadecyltrimethoxysilane self-assembled monolayer (ODS-SAM) on a Si substrate was investigated under the irradiation of vacuum ultraviolet (VUV) light of 172 nm in wavelength. The results of water contact angle, film thickness, and X-ray photoelectron spectroscopy analysis showed that the ODS-SAM decomposed gradually due to the VUV light exposure. Oxidized products, that is, -COOH groups, had formed before the ODS-SAM was completely decomposed and removed from the substrate. Coplanar ternary organosilane SAM microstructures have been successfully fabricated by employing the reaction between -OCH3 functional groups with -COOH groups on the VUV-modified ODS-SAM.