The surface molecular structures of Langmuir-Blodgett (LB) films of stearic acid with 1-12 layers formed on the hydrophilic substrates have been investigated in air by sum frequency generation (SFG). The SFG spectra of the even-numbered LB films were almost the same as those of the odd-numbered films, although it is expected that the molecular structures of the outermost layer for the even- and odd-numbered layers are different in the ideal model. In combination with AFM measurements, these experimental results are explained in terms of a flip-over model for the structural changes at the outermost layer of the even-numbered LB films.