Frictional forces between self-assembled surfactant monolayers on mica and a silicon tip were measured under dynamic shear, using a lateral force microscope. Molecular friction was resolved on the surfactant-coated surfaces. Dependence of friction upon load and shear velocity was also studied. The results are compared with those for bare substrate mica under similar conditions. We found that at a fixed shear velocity, friction increases with applied load in a nearly linear fashion for both the substrate mica and the monolayer-coated surfaces. Within the velocity range of 0.01 to 120 mum/s, friction increases monotonically with shear velocity for bare mica; however, for the monolayer surfaces, friction initially increases with velocity and then decreases. Similar behavior was also found for monolayers in hexane-saturated atmosphere, but with the maximum friction occurring at a higher velocity than that in dry air.