An alkylsilane self-assembled monolayer (SAM) has been micropatterned by the use of vacuum ultraviolet (VUV) light at 172 nm in wavelength radiated from an excimer lamp. By a chemical vapor deposition method, the SAM was prepared on Si substrates covered with native oxide from a precursor molecule of octadecyltrimethoxysilane. Micropatterning of the SAM was demonstrated based on a simple photomask-contacting method. Due to a distinct difference in friction coefficient between the VUV-irradiated and unirradiated regions, the photoprinted pattern was clearly imaged in friction force microscopy. The VUV-irradiated region showed stronger friction force than the unirradiated SAM surface. As confirmed by friction force microscopy, water contact angle measurements, and x-ray photoelectron spectroscopy, the SAM was found to be decomposed and removed due to the VUV irradiation. This photodegradation mechanism is ascribed to dissociative excitation of C-C and C-H bondings in the organosilane molecules and to subsequently proceeded oxidation with atomic oxygen radicals generated simultaneously through VUV excitation of atmospheric oxygen molecules. Microlines whose width was less than I Atm could be fabricated based on this VUV lithography.