Nonequilibrium molecular dynamics simulations were performed to study nanoscale friction between two Si(111) surfaces covered with alkyl monolayers immersed in liquid solvents. Three pairs of interfaces, ranging from hydrophobic CH3/CH3 to hydrophilic OH/OH, were studied. Three solvents, including water, methanol, and n-decane were used to represent different solvent polarities. It was showed that friction was dependent not only on surface hydrophobicity, but also on solvent polarity. In polar solvents (e.g., water), friction is much larger for hydrophilic than hydrophobic interfaces, while in nonpolar solvents, e.g., n-decane, friction has no significant difference for both hydrophobic and hydrophilic monolayers. A fundamental understanding of friction at different interfaces in various solvents is very important to micro- or nanoelectromechanical systems (MEMS/NEMS), particularly, bio-MEMS/NEMS. (C) 2003 American Institute of Physics.