The coverage of a self-assembled monolayer (SAM) of octadecyl silane on a silicon surface is analyzed by infrared external-reflection spectrometry via molecular orientation analysis. A commercially available octadecyl trimethoxy silane (ODS) solution is employed, which readily reacts with an oxidized surface of Si(100) prepared by a UV/ozone treatment in an ambient condition to yield a highly reproducible SAM. With the present technique, a very flat surface with no holes is obtained in the entire area of the SAM. IR external-reflection spectra of the SAM apparently indicate that the alkyl chains are highly ordered having the all-trans zigzag conformation, and the molecular orientation agrees with that of a Langmuir-Blodgett (LB) monolayer film of octadecanoic acid. Since the orientation analysis is based on an assumption that the surface is fully covered by the SAM as the LB film, the agreement of the orientation confirms that the surface coverage of the present SAM on Si(100) is on the same level as the LB film.