The frictional properties of self-assembled monolayers (SAMs) covalently bonded to silicon (Si) substrates through Si-C or Si-O-C bonds were investigated using a pin-on-plate tribometer. Four kinds of molecules with different alkyl chain lengths and different head groups were used: 1-hexadecene, 1-hexadecanol, 1-dodecene and 1-dodecanol. The SAMs with longer alkyl chains exhibited lower friction coefficients than those with shorter alkyl chains. Of the SAMs with longer alkyl chains, the SAMs with the Si-C bond of the head group exhibited lower friction coefficients, higher wear resistance and higher durability than those with the Si-O-C bond. The head group and alkyl chain length of the molecules affected the frictional properties of the modified surfaces of the SAMs. (C) 2008 Elsevier B.V. All rights reserved.