The photocatalytic decomposition of octarlecyltrichlorosilane (OTS) based self-assembled monolayer formed on TiO2 has been studied using atomic force microscopy(AFM), X-ray photoelectron spectroscopy (XPS), and contact angle analysis. The TiO2 thin films were grown on Si(100) substrates by atomic layer deposition from titanium isopropoxide and water. Densely packed alkylsiloxane monolayers similar in quality to those on SiO2 are formed on TiO2, It is found that the monolayers on TiO2 are decomposed much faster than those on SiO2 under UV irradiation of 254 nm in air. The OTS-based SAMs on TiO2 are decomposed through the photocatalytic oxidation of the alkyl chains with a gradual and homogeneous reduction in chain length. After the complete photodecomposition of the OTS-SAMs, the siloxane headgroups remain on the TiO2 surface. The observation indicates that the titanium oxide, a well-known photocatalyst for organic pollutant treatment, efficiently decomposes the alkylsiloxane monolayers under UV irradiation in air.