Monolayer structures of a chiral mesogen, (R)- or (S)-[4 '-(1-methylheptyloxy)-3 ' -nitrobiphenyl-4yl] 4-(trans-5-decenyloxy) benzoate, adsorbed on graphite were investigated with molecular dynamics simulations to clarify the mechanism of chiral supermolecular structure formation. We proposed a mechanism model that is based on the coupling between up-down symmetry breaking and the molecular tilt direction symmetry breaking of the monolayer.