A long-chain dialkylated olefin, 2C18 was found to form a close-packed, highly insulating self-assembled monolayer (SAM) on Si(111) via Si-C bond formation. The IR absorption spectra of the 2C18-SAM show that the alkyl chains are densely packed with an all-trans conformation. The threshold voltage to initiate the scanned probe oxidation (SPO) of the 2C18-SAM-covered Si(111) was ca. 9V, which is much higher than the previously reported voltages for organic monolayers on Si or SiO2/Si substrates.