We build up a self-consistent equation and derive a general equation of orientational phase transition between a normal-director phase and a tilted-director phase for smectic monolayers on a water surface under monolayer compression with a consideration of molecular configuration. A differential dielectric constant for monolayer films is discovered and discussed. The change of the first orientational order parameter and the dielectric constant at the polar orientational phase transition point is also examined. It is found that the influence of the polar orientational phase transition on the first orientational order parameter in monolayer films is clear compared with that on the dielectric constant. This reveals that Maxwell-displacement-current measuring technique would be a useful way to measure such a phase transition.