Molecular dynamics simulation is used to study the friction of mixed Langmuir-Blodgett film composed of hydrocarboxylic acid (CnH2n+1COOH, n=12,13,14,15,16,17) and the C17H31COOH, in order to investigate the influence of the structure on friction. The results show that the restiction of long chain molecules by the neighbor molecules increases and the monolayers become more steady as n increases, while the shear pressure between the monolayers decreases and reaches a minimum when n=17. The hydrogen-bond formed in the mixed monolayers is the main reason why the monolayers become more steady, espatially,the hydrogen-bonds are most steady when n=16, but the monolayer composed that just of C17H31COOH molecules has the best sliding effect; there is no hydrogen-bond between the two monolayers. The shear pressure is hardly affected by the molecular deformation.