We report on nanolithography using a scanning probe microscope (SPM) as an exposure tool of an organosilane resist consisting of a trimethylsilyl monolayer. This monolayer was prepared on silicon substrates by chemical vapor surface modification with hexamethyldisilazane. The SPM exposure of the resist was conducted under air, nitrogen (N-2), and oxygen (O-2) atmospheres. The trimethylsilyl monolayer was locally degraded along a scanning trace at a sample bias of +5.0 V. The degradation of the organosilane monolayer was induced irrespective of the atmosphere. This degraded region served as an etching window in an aqueous solution of ammonium fluoride and hydrogen peroxide. Consequently, etched grooves of 80 nm in width were fabricated. The degradation mechanism is discussed in terms of SPM tip induced anodic oxidation.