The friction and wear properties of silicon surface covered with octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) were investigated by a UMT-2 microtribometer with and without water as lubricant, and then compared with that of bare silicon surface. Dry friction measurement results show that OTS SAMs have a very low friction coefficient compared to bare silicon surface under lower sliding velocity and normal contact load. However, heavy wear occurs on OTS SAMs under higher contact stress and sliding velocity. Under water lubrication, OTS SAMs can prevent wear obviously and meanwhile present low coefficient of friction even under high velocities. The improved frictional and anti-wear property on OTS SAMs surface is attributed to the hydrophobic property of OTS and hydrodynamic effect of water. Furthermore, a wear critical phase diagram for OTS SAMs with and without water was proposed, which indicates that OTS SAMs working under water lubrication owns a wider range of available load and velocity to reduce friction and prevent wear. The friction and wear properties of silicon surface covered with octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) were investigated by a UMT-2 microtribometer with and without water as lubricant, and then with that of bare silicon surface. Dry friction measurement results show that OTS SAMs have a very low friction coefficient compared to bare silicon surface under lower sliding velocity and normal contact load. However, heavy wear occurs on OTS SAMs under higher contact stress and sliding velocity. Under water lubrication, OTS SAMs can prevent wear ... and meanwhile present low coefficient of friction even under high velocities. The improved frictional and anti-wear property on OTS SAMs surface is attributed to the hydrophobic property of OTS and hydrodynamic effect of water. without water was proposed, which indicates that OTS SAMs working under water lubrication owns a wider range of avail able load and velocity to reduce friction and prevent wear.