The polarization dependence of electric field for hemicyanine layers deposited by the Langmuir-Blodgett (LB)technique has been measured. The experimental results show that ferroelectricity exists not only in thick films(200nm) but also in thinner films (30nm), and the remnant polarization is related to film thickness. In order to interpret the measured results, a planar rotor model was introduced, and a relation between polarization and film thickness was obtained by perturbation theory. The theory fitting agrees with experimental results well. It is confirmed that ferroelectricity in organic molecular LB films mainly arose from altering of molecular orientation.