Molecular dynamics simulation has been used to study the friction and structure mechanism of Langmuir-Blodgett films of hydrocarboxylic acid (C15H31COOH) deposited on the SiO2 substrates at different sliding velocities. The results show that, for a certain monolayer, the shear pressure between the monolayers increases when the sliding speed is below 60m/s and decreases when the sliding speed exceeds 60m/s. The molecular tilt angle decreases as the velocity increases. The molecular clusters formed in the monolayers by the hydrogen-bond caused the long shear periods, but there is no hydrogen-bond between the monolayers.